Horrible girlfriend and why you stayed for so long??????

Okay I have to know this. Why is it that some guys who have the worst girlfriend imaginable, I mean the equivalent to a boyfriend that beats his girl but the feminine version, wont let them go. Even though you know that you wont stay with them long and that there is nothing between you two and you know you dont love her and that she really isnt anything special but you still remain with her. And she isnt pretty or hot at all. Okay now someone please tell me what would keep you holding on when everyone else tells you to break up.