Why do guys cheat....?

on their longterm girlfriends with loud obnoxious types?? i am quiet and reserved, the guy i was with 6 yrs headed off saying he was bored.. when i contacted him there was always some loud crazy chick , laughing in the background.i mean REALLY loud!! 2 months later he is back with me and so sorry and attentive. now there is no trace of this woman.I mean what happened here???