Why do men cheat ??

Nothing to do with a problem of mine. But it's out of total curiosity and has been for a long time. I've looked it up online and stuff and it always says "he's too insecure" like is that true ? 
Why do guys cheat ? And what makes a cheater go back to cheating when he has a perfectly good girlfriend ... ? 
Just wanna know ,post what you think 🙄🤗