how could he act this way?!

before people attack me for this, I am done getting involved with my "crush" on him cheating, I confronted him about it before yesterday and all he says is that he's just going with the flow and I told him that I knew that he won't ever tell his girlfriend that he's cheating on her on his own because I've told her plenty of times she wouldn't listen to me.. why is he cheating on her? he's been cheating on her since the beginning of their relationship for the past year! will he ever get his act together?! why won't he ever tell her the truth!