unattractive men cheating on beautiful girls?

Why does this happen? Me and my (not so good looking) ex had a beautiful relationship until he cheated on me.

By the way im not trying to sound mean saying that about him but it is true, i fell in love with HIM ,not his looks.

He would always say ‚How are you with me‘ and that im way out of his league. Plus i was the best i could ever be for him i was the most nicest,loving and caring girlfriend