Most confusing relationship delima of my entire life :(?

As a girl, I often wonder why girls who have a perfectly nice boyfriend will go for the guy who's hurt them, abused them, or is a general douchebag. What is the allure there? I've seen guys do the same thing, but not to the degree that girls do. Maybe it's a self-esteem thing. Stay with the nice guy. Realize that you left the other one for a reason, save yourself the heartache.