Just a simple why, preferred answers from cheaters not so much opinionists.
Most Helpful Guy
Can be both, there's no hard and fast reason which fits all circumstances. Some guys would prefer to have sex with their wives/partner, but for whatever reason the partner has decided that they won't have sex with the guy.
Sometimes a guy is just a serial cheater because he doesn't see the point of monogamy, and never will.
Sometimes the guy is just a guy who thinks that if no one will ever find out, then it doesn't really count.