Whys is it that men always treat a good woman badly & then come back years later?

I always find that in relationships I am good to a man & have great qualities. For some reason, men always reat me badly or cheat. When I set my boundaries & say enough is enough & break up with them. They go out there & date looking for something better but maybe 2- 3 years later - they come back begging, wanting me back. They realize that the gr is not greener on the other side. By that time Ihave no interest in them at all. Not that I think they are bad, but because I realize I can do better - I let them go. Actually, I find myself happy that I was rejected because I would have ended up with a jerk. Why can't men realize a good woman when they have her. Is it immaturity or are they so uncontrolable that they have to constantly look for better when they already ahve something good?