Wonders of human relationship?

I've always been pondering why some people act the way the act in relationships.. what actually drives them to do such things to someone they actually put effort (full or not) to build a relationship with. When betrayed, how can a person still hope and believe in the one who betrayed them? How can someone put so much effort to go after someone and once they feel bored, they would just ignore them?

Well, i have always criticized a friend of mine for being extremely idiotic and stupid because he still have faith in a girl who dumped him for someone else.. although i do not know 100% what actually happened but the truth is that his relationship with that particular girl is over and although as much as he still hope that there is a chance of getting her back, its pretty....bleak? He still clings on to her and never seem to give up on her.. for me, i do not understand why would he chose to close his heart for that one girl that left him? what does he hope for to happen? i know im don't have the right to judge him but still, why can't he get the picture? well is that true love or just plain denial of the past? i really hope that he would snap out of it and face the future instead of dwelling in the past..