Stupid mistakes dont usually have a 'why' to back them up. And if they did, it wouldnt be one that is satisfactory enough to make you feel better. Some people are just selfish, bad people.
my best friend is in this situation... she can't seem to let go of this douchebag that did her wrong. She's engaged now, so that helps, and she stalks him randomly on facebook---but usually only once in a few months. it does get better.... don't worry.
Sometimes you will never find out WHY someone did what they did, maybe they do not even know why themselves. Or maybe....you are better off NOT knowing why they did what they did.