Most Helpful Girl
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A lot of those girls have a "messiah complex" and feel like they need to "save" that kind of a guy. I dated a girl who had this problem and she was turned off by good guys. I was good to her, and when she noticed this she openly told me that when she meets a good guy she gets turned away.
So having met a girl in such a scenario I can tell you that they don't last in long relationships and are very easily manipulated. They're quite giving in nature but also have an attraction to the drama that such a relationship brings so it is very conflicting. But if they do end up fulfilling their goal the only thing they get is that 1 moment of gratification if the guy does "change" and he pours out all these "feelings" onto her. She'll get a rush from the thrill of "turning him into a good guy" but then be left hanging and eventually dump him because her "mission" is over. There's no more "struggle" for her.