when a man cheats, why does he want to come back?

I have been going out with a guy for almost 2 years now and I had a suspision he was cheating (we don't live together) and it was pretty much confirmed yesterday. What I want to know is why a man cheats but still wants to come back to you? I mean if a guy or gal moves on to someone else for sex why would they want to stay fooling around with the one they left to go cheat on in the first place?
I am confused as to why he still wants to see me if hes been with someone else,