Most Helpful Girl
Bad habits and a history of being spoiled/getting away with things.
Or maybe she hates the girl who wants to get back with the guy she's hooked up with and want to destroy her life. Or, two, she wants to destroy the hopes of a girl who's been working really hard to get the guy she's hooked up with.
There's also the possibility that guy she's hooked up is her bank for everything she needs in life so hey, what better way for him to feel he's responsible for her than to resort to such a filthy method. Guilt tripped. She's just the type of girl who wants more than she can swallow.
And the last: she's a very scary person and the guy she's hooked up with should realize that soon and run before he's chained/caged by her for eternity.0