"Good Girls" and the "Thug" magnet.

What is it. I am talking about true bad boys, real live thugs. I have heard too many stories about these guys finding a nieve young lady and practically ruining her life. I have seen this happen to girls with strong backgrounds (college, two parent household, love, discipline, you name it). I have also seen it happen to girls with rough upbringings. It does not matter. There is something about the bad boy, or thug that takes power over these ladies. I wish I knew the answer. Can anyone give some insight.