1. Moments of connection. It's hard to tell, but all in all, I think the single greatest draw for me in pairings is when the characters share moments of intense connection. I'm extra attracted when these moments start out as non-romantic in nature and then turn romantic later on, but I can get into a pairing that's romantic from the start if they've got moments like these; sometimes I can even start shipping a pairing that's not romantic at all in canon when they connect amazingly.
When I talk about "moments of intense connection"--it's hard to explain exactly what I mean except by giving examples. I mean things like Athrun and Cagalli trying to kill each other but ending up caring for each other instead, Miriallia's first scene with Dearka, Rukia putting her sword through Ichigo's heart, even, although it's not as intense as any of the above on the surface, Temari rescuing Shikamaru. Instants in time where two characters just utterly and unexpectedly click.
When it's more expected because of preexisting romantic ties--for instance, Sakura hugging Sasuke in the Forest of Death--I'll still be touched and intrigued, but it won't thrill me quite as much.
Examples: Dearka/Miriallia, Athrun/Cagalli, Temari/Shikamaru, Rukia/Ichigo, Saber/Shirou. It's hard to find pairings like this, but when I do find them, I latch on. (Actually thinking of Zangetsu/Ichigo as falling into this category--which it does--gets me more interested in it. Hmm!)
2. Intensity. I alluded to this in the previous reason when I specified moments of intense connection, but it really deserves a listing of its own. Any pairing where the emotions involved are deeper, truer, and maybe crazier than anything normal really gets to me. I guess it's just my twisted way of being a romantic.
Examples: Yoruichi/Soi Fong, Sasuke/Sakura, Yoruichi/Soi Fong, Squall/Rinoa, Hawkeye/Mustang, Yoruichi/Soi Fong, Naruto/Sasuke, Allen/Rinali (?), Yoruichi/Soi Fong
3. Ties that bind. Another thing that really draws me into pairings is when the characters involved are in some way bound to each other, by fate or duty or promises or something--sometimes I'll perceive them as bound together and somebody else won't at all. But the important thing is that sense of spiritual bondage. Mmm, bondage. This is especially reflected in my threesomes, which are only sometimes necessarily romantic in nature.
Examples: Yoruichi/Soi Fong, Hawkeye/Mustang, Sasuke/Sakura, Naruto/Sasuke, Kakashi/Sasuke, Naruto/Sasuke/Sakura, Jiraiya/Tsunade, Kakashi/Rin, Kira/Lacus/Athrun/Cagalli, Allen/Rinali (?), Athrun/Kira, Seifer/Rinoa/Squall, Nooj/Paine, Clair/Nel
4. Character growth. It makes me go squish when the relationship between two characters makes them change and grow for the better, especially when it's romantic in nature. However, unlike pretty much all the other reasons, for this one to snare me, I have to already be in love with one or both of the characters. Normally pairings draw me in independently of the characters involved--they function almost as separate entities in their own right--but in this case, it's all about the characters. Occasionally, though, I'll go awwww over a pairing for this reason even when I'm not in love with the characters involved. It's hard to predict.
Examples: Sasuke/Sakura, Naruto/Sasuke/Sakura, Ino/Chouji, Kira/Lacus, Mwu/Murrue, Edge/Rydia, Ichigo/Orihime
5. Unresolved sexual tension. Okay, so this one's pretty much a gimme. If the source can convince me that the two characters in question are humming with enough barely contained sexual energy to power entire citystates, I'm hooked.
Examples: Hawkeye/Mustang, Temari/Shikamaru, Yoruichi/Soi Fong (see, this is why I don't like them having had sex prior to Yoruichi's leaving Soul Society. Because then it wouldn't be unresolved), Chizuru/Tatsuki, Dearka/Miriallia, Athrun/Cagalli, Nooj/Paine
That was harder than I thought. But now I'm done.
(The mentions of Allen/Rinali get question marks because it's been so damn long since I went over that manga and it didn't really stick very well in my brain the first time. Must reread soon! But first I should finish that damn Ichigo/Orihime essay. And de-SEED my brain. Yeah, like that's gonna happen.)
I miss my muse. I wish it would come back. I have all these ideas that I want to write and I'm too blah to do anything with them. (My body is obviously conspiring against my brain. I finally sit down and start writing some Mwu/Murrue, and bam, I get sick and lose my inspiration.) At least I have antibiotics and codeine now!