The way Mabel sees it, there's four good reasons for having Bill Cipher around.
One:
She loves to entertain the idea that somewhere deep down in that blackened heart of his he enjoys being near her. Why else would he constantly be sneaking into her room at late hours? Just for a, "Hello shooting star."
Maybe he enjoys her company. Maybe she makes him feel something that she nor him can pinpoint, but it's there. And the thought him enjoying her company made her feel somewhat appreciated. Like he considers her more of an aquaintance than someone who he just loves to mess with and, man, did he like messing with her. Whether on purpose or completely by mistake, there'd always be something he'd do to make her feel like it was all just a dream.
He'd show up at her window, she'd let him in. He'd smile or say hello and even grab her hand placing a delicate kiss on her heated skin. She swears she could see a tint of red running over his face.
But Bill doesn't blush and she'd never admit to hoping he would.
Two:
He may be evil, he may be cinical, he may be the worst thing to ever happen in her life, but he's become more like her, and dare she say she's turning into him.
He's wormed himself into her life and it didn't take any special deal of his to do it. Just a few nights alone in her room, chatting about everything and nothing at all, reminiscing on her summers in Gravity Falls, things he's done in his life. Things she couldn't imagine. And those times where they couldn't say anything at all, he wouldn't leave. He'd stay and wait until she couldn't stay awake anymore and he'd be gone by the time she woke up.
Sometimes she'd hope he'd still be there.
Three:
Being with Bill gave her a rush. Exhilerated by this man she barely knew, even if he told her everything. He's still him and she'd always see him as someone who could harm her at any moment.
"I could set fire to the whole damn world if I was invoked to," he'd grin wickedly followed by an unerving chuckle. She's mile back until she realizes that he actually could. He's probably even get a laugh from it. Yet for whatever reason he doesn't. She'd like to believe she had something to with it. Almost as if being around her made him calmer.
She didn't know if it had any truth behind it and she'd never bring herself to ask.
Bill's one of a kind. That's the only real thing she knows about him.
Four:
It was a hurtful thing to realize. It stung in ways she never thought was possible, but it does. The only thing able to sooth the pain is ironically him.
The last, and the most outragious, being that...
She might actually be falling for the descpicable baastard.