A friend of mine--someone I NEVER thought would cheat on her husband--did. It shook up our whole friend circle.

She told me (later, after their divorce) that she thought there were 2 reasons someone cheated: Either they were going to be a serial cheater, and no amount of affection/attention/adulation would ever be enough (because it was a hole or deficit inside them they were seeking to fill); OR in a cowardly attempt to self-sustain or survive until they're ready to leave a situation they are no longer all in.

Incidentally, she's been remarried for 6 years now and has never once thought of stepping out on the guy. She thinks she's in camp two. I hope, for his sake, she is.