Why is it that some couples cheat on each other?.

You would have to ask the couples that reason. I know a guy that cheats on his wife as she cut him off from sex.

Sometimes it happens at the beginning of a relationship when it's not that serious yet. An ex of mine cheated on her new boyfriend with me. She told me that she didn't love me but didn't love him either. It happens all the time. Playing one guy or woman off against another; wanting to have their cake and eat it; call it what you will.

Apart from that I'm not sure but my theory is that people cheat because it's exciting and there's the risk of getting caught. It's like shoplifting or Hugh Grant going with a cheap hooker in a parking lot, when he could have got a hotel room, or a high class prostitute where nobody would have known and no questions asked.

Selfishness!

Attention. Ateast that was for my last boyfriend a couple years into our relationship