It sounds like you understand the inherent dichotomy of the single female psyche. They say they want a nice guy, but they always go out with these total losers. Then, what's worse, they complain to you about what a jerk LoserBoy is, and wish that they could find a 'good man'. Well, Dave, Jonny here has figured out the secret. When they are 30 or so, their tune will change.
Nice guys are too easy.
You see, if she can get him to change, that must mean that he really loves her. If you don't believe me, check out some of those romance novels. They always feature some guy who is really a swine, but Miss Paula Perfect changes him into a devoted father and husband. This, of course, irritates the female to no end. She continues to hang in there, hoping that she can change him again.