Gratification
Lily doesn't like him the first time she kisses him.
And she doesn't mean that in an opposites-attract, sexual-tension kind of way. It isn't him she finds attractive: his nose is too long, and his eyes are too close together, and the rumpled effect just doesn't look good on his hair, try as he might. As far as she's concerned, anyone will do—and it just so happens that James is the one to catch her alone in the common room right before she snaps.
And it's not that she has low standards, either. Lily's a particular sort of girl (at best, a perfectionist at worst; she gets in over her head, so get out of her way). Admittedly, this is her own doing. No one forced her to stop going out after Severus left her—or maybe she left him, she's not quite sure, the lines blur—everything blurs when it comes to Severus. If he had only been a mate, though, why did losing (leaving?) him prompt her to dump her then-boyfriend and stop dating for a good year and a half? She told herself she wasn't ready to put her trust in someone so fast after what she'd been through, but then, she's always thought that boyfriends aren't to be trusted, only to be toyed with.
It's been months since her last kiss, but at this rate, she knows it'll be hours (or less) until her next. She's feely antsy but aroused, as is usual these days, and is altogether ready to throw in the towel and quit her studies for the History of Magic exam tomorrow, quit responsibility, quit—all of this, the trials she puts herself through. She'd be the first to admit that she's not very nice (everyone else seems too busy calling her witty and charming to care), and maybe that means she doesn't deserve a much-needed break from her life, a bit of relief, anything. Even so… maybe she's particular, but honestly, just give her half an hour in a broom closet with a cooperative bloke to tide her over. Anything.
Lily's not quite sure exactly how it happens—he's using some awful pickup line one minute and thanking his lucky stars the next. She knows she must have initiated it, because James would never be brave enough to come onto her with so little chivalry, and she's pretty sure she told him to shut it a few times when he was trying to make sense of the mixed signals she's giving. But frankly, she can't be bothered about his feelings or, God forbid, tomorrow just yet. Lily Evans has an itch to scratch, and she doesn't intend to allow for any distractions until she's made up for the last year and a half without touch.
Besides, to his credit, he's much more tolerable when they're snogging, as she doesn't have to look at his flaws or listen to his folly. (To her credit, she never pretended to be nice.)
They're not kidding when they call it "falling in love," Lily's going to find out, because she can't imagine just yet how the hell she'd ever stumble into it with a plaything, a cowardly lion, like James Potter.