There's something deliciously self-destructive about that toerag. He's everything she never wanted. He smokes too much, he swears too much, he sleeps around too much, he fights too much, he gets caught too much, he gloats too much, he -
He is in Lily's head too much.
Like a scrape that won't heal. When you've only just forgotten, he turns up again and aggravates the wound. It hurts, a lot. And often.
She tries her best, really. To get along. But he always makes it worse, finds a new scrape to poke at, and it hurts all over again. She doesn't like not getting along. She's not like that. She gets along with everyone. But not with James.
If she's honest with herself, the poking hurts her feelings, but the not-getting-along hurts her self-image.
"Princess," he'll sneer at her, like he's the only one in the whole world who knows she isn't perfect, like he's the only one who sees through her bullshit. (He'd get along exceptionally with Petunia.)
Sometimes she thinks that maybe it is bullshit - all these prefect meetings and perfect scores on homework assignments and going to bed at 10 p.m. - why do they matter if she doesn't even know if she'll have a future? If her best friend can call her a mudblood and You-Know-Who can murder people just like her, who says she'll even live long enough to put all her school achievements to use in a proper career -
Does she even want that?