I had an old friend who was a chronic cheater. He would get very entrenched in a serious relationship, fuck it up by cheating and move on with the new broad. He lost a house. An engagement. He knocked up a married chick who already had two kids. Blew up her marriage and his own relationship. Just kept going down the same path over and over. He's 45 now with a young wife and a bunch of kids from god knows how many women, probably still living that same life.
I don't know what causes this but it seems insane to me to take on and continue creating this much damage, yet a ton of people do it. /shrugs