Well...they have to clear their conscience. See, every girl has to make a massive screwup in their life. Almost no girl I ever met around my area has liked stability. They make these screwups, so that there are problems to deal with and stress over. THey get ****headed boyfriends for the sport of it--to see if they can tame the ****head. Which of course, they can't.
Now the whole process works out really beautifully, cause they can date the ****headed guy for however long they want to and enjoy pretty mindless fun. They don't have to worry about romance or commitment because things are usually too ****-up-ified from day one for those things to even realistically be possibilities.
From the moment they start with that guy, they get to do things like believe in him and say "No, I know the real him!" and "He loves me!" Then it generates conflict. THe conflict is a game; so the girl can kill time between getting slammed.
The male end of this is fascinating, actually. If you are a decent guy, then a girl will use you completely, like they do to me. Not "use" in the traditional way, though. What they'll do, is talk to you to fill in the void of emotional and intellectual stimulation that ****head Mc****fest can never provide. After a few quick hits off the "real friendship pipe" they can go back into the world and act like douchebags again without having it on their conscience.
(Now bear in mind, this is applying to all the girls I've met, just about. But I haven't been many places other than my home state...so hopefully my findings are not true of them all.)
Girls are seemingly genetically incapable of being satisfied. It is very difficult to find one with that ability. Even the nicest girls I know need constant gratification. Like Nelo; in your case, I bet she just got bored. That's why girls need to have a jerkoff who fights with them all the time; cause in our ****ed up generation, all the girls seem to be mentally damaged in one way or another, and they need to have some sort of conflict going on in their relationships because that is the only way they stay interested: if they have to work for it, and fool themselves.
So long as losers like me provide them with the deeper end of things in small enough doses, anyhow.
In my experiences, they usually know damn well when they start dating a decent guy and everything's fine, that they're going to end up leaving him. They'll never say it to your face, but usually, there are minor red flags--"I don't want to hurt you" rings a distant but familiar bell.
It's all part of The Game, man. Sometimes I am damn glad that I'm not involved in that crap anymore.