So what is this? This, my friends, is a guide to losing a pairing debate.
Let's start with the worst arguments. If you use any of these points in your argument, the debate is over. You lose.
1. "Character A will only ever love Character B; for her to wind up with anyone else would be settling!" This relies on a rather limited vision of love. In reality as in well-written fiction, people are capable of falling in love with more than one person over the course of their lifetimes without having to "settle" for anyone.
"Sakura can't ever be with anyone but Sasuke; she'd just be settling for them!"
"Sasuke can't ever be with anyone but Naruto; he'd just be settling for them!"
"Naruto can't ever be with anyone but Sakura; he'd just be settling for them!"
2. "Character A belongs with Character B, because Character C sucks!" If you have to bash the perceived "opposing" character to justify your pairing, you're really running low on actual relevant arguments. Next!
"Yoruichi/Soi Fong is out of the question, because Soi Fong is a bitch! Urahara/Yoruichi all the way!"
"Rinoa sucks, so Squall should be with Seifer instead!"
3. "A/C will happen, because B belongs with X!" I call this one "sideshipping." Some call it "character dumping." Whatever it is, it's really stupid. Like argument #2, bashing the opposition, sideshipping shows an insecurity in one's own favored pairing. Why do you need to bolster it by pushing other characters around in unlikely ways?
"Sasuke and Hinata are meant for each other. Therefore, Naruto and Sakura will be together!"
"I love Al/Winry and Havoc/Hawkeye! Oh look, that leaves Roy and Ed free for each other! What a delightful coincidence! See how perfect it is?"
4. "Character A is so awesome that he belongs with anyone!" Some characters pair up well with lots of different people. That's one thing. (It's also a matter of personal preference.) But when you're arguing that Character A is so wonderful that he should get anyone and everyone? That's another thing, and not so cool. No matter how much you love a character, there are people they'd do well with and people they wouldn't.
"Hinata is the cutest, most wonderful girl ever, so it's only sensible to pair her up with Neji, Sasuke, Itachi, Kiba, Shino, Naruto, Kakashi, Sakura, Ino..."
"Naruto is so awesome that he should be doing every girl in Konoha!"
5. "I have no evidence to back this up, but Character A and Character B belong together!" This is called an a priori assumption. It's a logical fallacy. Don't do it.
"Ino now has a crush on Shikamaru instead of Sasuke."
"Athrun and Lacus have always been in love."
6. "Character A is the male lead/hero and Character B is the female lead/heroine--they have to wind up together in the end!" These are false expectations brought on by too much exposure to cliched genre flicks where the romance is pastede on yey and to cliched romance novels where the plot is pastede on yey. In well-written fiction where the romance is balanced well with the rest of the plot, the hero or heroine might wind up with the most prominent character of the opposite gender, or they might not. It can go either way.
"Harry is the hero, and Hermione is the heroine--if you say otherwise you're bashing her! And since Harry is the hero and Hermione is the heroine, they are the only acceptable pairing in the end."
"Rukia is the female lead. Of course she's going to wind up with Ichigo."
7. "Character A is meant for Character B, therefore Character X is meant for Character Y." In other words, taking one set of character interactions and applying it to unrelated characters. People will sometimes go so far as to compare characters from entirely different series in this way. But unless the author has explicitly drawn parallels between different characters and their situations, made comments about the influence of such-and-such a series on their work, or very clearly set their creation within a genre with strong romantic conventions (i.e. the Gundam series and their tradition of killing off the hero's first love interest), such comparisons are meaningless. Even then, it's a tricky business and shouldn't be relied upon too much.
"Sakura is in love with Sasuke, so Ino is in love with Shikamaru and Tenten is in love with Neji!"
"Winry throws wrenches at Ed and they're a couple, so when Rukia hits Ichigo, it means they'll be a couple, too!"
8. "Look at what happened in the filler arcs/my favorite fanfiction!" You'd think most people would know not to point at filler arcs or movies in anime for examples of why a pairing works or doesn't work, but amazingly, some people still do it. Filler arcs frequently have little to do with how the creator of the series envisions the characters. It's like, to move on to this argument's twin, using fanfiction to justify a pairing. Sadly, this happens too in pairing debates.
"Neji and Tenten worked together so well in the Curry of Life arc--they make a great couple!"
"There's this awesome fanfic that proves that Harry/Draco works perfectly as a pairing. Read it and see!"
9. "Character A deserves Character B more than Character C ever could, so he should get her!" This is my single most hated argument. Say it with me: people are not prizes to be awarded. The matter of deserving should not enter into consideration of a couple, in real life or fiction. If two people have a healthy, loving relationship, it doesn't matter whether one of them is a saint and the other flawed and disagreeable. Romance is not a merit system.
"Naruto has always been much nicer to Sakura than Sasuke ever was, despite being through just as much pain and suffering! It would be a bad moral example if he didn't get her."
"Meyrin is so sweet and loving! She deserves Athrun after all she's done for him."
10. "Character A is so important to Character B that B could never have a romantic relationship with anyone else." I have to wonder if this is a peculiarity of modern Western culture, the idea that people can't be important to each other without there being a romantic element to their relationship. Sometimes this even spills over into familial relationships as well, with people insisting that if two siblings are unusually close, they must be paired with each other or no one else at all. In reality (as in well-written fiction, as I keep saying), though, it is possible for someone's best friend or brother to remain a profoundly important force in their lives even when or if they hook up with or even marry someone else.
"Ed and Al are so important to each other and have been through so much together--the only way they'll ever have a happy romance is if they sleep with each other."
"Sasuke could never love Sakura as much as he loves Naruto, so the pairing would never work."
Next are the mediocre arguments--ones that are acceptable as personal explanations of why one enjoys a pairing, but not as objective points backing it up. These can be mentioned as minor issues in support of a pairing, but if they are held up as the bulk of the argument, the debate is over. You lose.
11. "Character A looks good with Character B! They're so pretty!" Yes, yes, the pretty is nice. However, in the long run, it says nothing about whether or not two characters make a good couple.
"Don't Sasuke and Sakura look beautiful together? They're meant to be!"
"Roy/Ed is so hot! OTP!"
12. "Character A is always standing next to Character B! They must be having sex on the side!" It's funny, but you generally only see this argument when the two characters in question are, you guessed it, pretty. If combined with lots of other hints, the fact that two characters always show up near each other in canon or in official art might mean something, but on its own it's irrelevant. Maybe the artists just like how they look together; maybe the actors just play well off each other. Who knows? Romance is certainly not an inevitable conclusion.
"Look at Neji always standing by Tenten's side! It proves he loves her."
"Dearka and Yzak are always next to each other in official art, you know. That means the creators of SEED support Dearka/Yzak."
13. "Character A and Character B argue all the time. They're so married!" Often, bickering between two characters can indicate that there's also romantic tension there. Sometimes, though...it doesn't. The presence of good-natured squabbling alone doesn't mean anything unless the writer somehow indicates that there is also a certain amount of romantic interaction.
"Ichigo and Rukia bicker like a married couple. IchiRuki is canon!"
"Roy and Ed argue all the time. Can't you just feel the sexual tension?"
14. "Character A and Character B argue all the time. They should never get together--it'd be abusive!" On the flip side of the coin, just because there's conflict in a relationship doesn't mean that it'd be doomed as a romance. Some people enjoy that sort of thing in their lives; others consider friction worth it to maintain a relationship with someone they love.
"Athrun and Cagalli had a rocky start to their relationship, and they still argue. It's unrealistic for them to get together as a couple."
"Ron and Hermione hurt each other all the time. Their relationship is doomed!"
15. "Insert overblown metaphor improbably justifying A/B here." If you can come up with pretty metaphors and symbolism for your favored pairing, good for you. But it proves nothing and, ultimately, can often be twisted around any which way.
"Naruto is the Earth and Sakura is Heaven. They complete each other."
"Harry and Hermione's ride on Buckbeak symbolizes how the purity of their love will save the world!"
16. "Character A only has a silly crush on Character B. She'll get over it and become stronger that way." Funny, but this argument is used almost exclusively on female characters. Hmm...anyway, the perception is that if their feelings for another character started out as a shallow crush, they must always remain that way. Sometimes this is the case, but frequently this argument ignores evidence that the character's feelings have grown over the course of the series. It also denies the possibility of a character being able to become strong and stand on her own while still having feelings for someone else. Cutting off all romantic ties is not the only path to emotional stability.
"Sakura's feelings for Sasuke were a silly schoolgirl crush. Now she's grown up and knows better!"
"In order to mature as a character, Orihime has to get rid of her obnoxious feelings for Ichigo and learn her place."
17. "Character A can heal Character B's tormented soul!" I guess this has something to do with hurt/comfort as a genre--the idea that love is about healing, and usually one-sided healing at that. Unfortunately, when you actually step back and look at relationships, they don't really work this way. It takes a lot more than a single person with a Loving Soul, a Gentle Heart, and the Smile Of An Angel to work through someone's damage.
"Hinata is such a sweet, caring girl. She could soothe the darkness in Itachi's soul."
"Sakura is a healer. So she could heal Gaara's pain by having sex with him!"
18. "Character A understands Character B's pain, so she's the only one for him romantically." It's one thing to enjoy a pairing because of the characters' shared or similar experiences. It's another to insist that it's the only acceptable pairing. Just because someone shares difficulties with someone else doesn't mean that they can't find romance with another person entirely. They might not even be attracted to the person who "shares their pain," after all.
"Neji and Hinata both suffered because of the way their clan is run. Therefore, they must turn to each other for the best romantic relationship."
"Naruto and Gaara both know the pain of being a jinchuuriki. They are destined for each other."
19. "Character A and Character B complement each other." This nebulous reason is often trotted out in favor of pairings when canon evidence is lacking. Sometimes, it's couched in the terms of "opposites attract." Unfortunately, it doesn't really mean anything. So two characters have personalities that some fans think would mesh well. That's nice, but unless you can provide solid examples, it doesn't really mean anything.
"Naruto is outgoing and Hinata is shy. Opposites attract!"
"Ichigo is angry and protective, and Orihime is sweet and wants to be protected. They complement each other!"
20. "Character A and Character B are equals. That means they're the best partners for each other in romance as well as everything else." Sure, lots of people have a thing for pairings where the characters are partners in all things. But it's important to remember that this is a preference, not an absolute. Yes, you do need a certain amount of equality for a romantic relationship to work, but you don't need to be the one person who fights at your lover's side in all matters. It also doesn't say anything about whether or not Character A is even romantically interested in the person who happens to be his partner and equal. You want to say that he is? Fine, but don't insist that it must be true.
"Look at the way Urahara and Yoruichi fight side by side. Urahara/Yoruichi is the best possible pairing."
"Naruto and Sasuke kick ass together. Neither of them could ever be with anyone else romantically!"
So if those are the ways to lose a pairing debate, how do you win one? Sadly, you don't. This is the interweb, after all. We're all retards in the end! :D But if you avoid using fallacious arguments, you can at least keep your self-respect. Right?