Humans are hard-wired for short term rewards, so I think a lot of cheating happens just because the opportunity comes up. That's probably responsible for most of the cheating out there.
For the rest... despite fairy tales of "happily ever after", its common for people to be dissatisfied with in a relationship. It could be temporary, or it could be long term. It could be a fundamental incompatibility, or loneliness, or it could be the result of forcing oneself into a relationship mold that is a bad fit. It can be hard to tell a long-term partner that you need something he/she is not able to offer - and for most people its much easier to "cheat" than to just have an honest talk.
For the rest... despite fairy tales of "happily ever after", its common for people to be dissatisfied with in a relationship. It could be temporary, or it could be long term. It could be a fundamental incompatibility, or loneliness, or it could be the result of forcing oneself into a relationship mold that is a bad fit. It can be hard to tell a long-term partner that you need something he/she is not able to offer - and for most people its much easier to "cheat" than to just have an honest talk.

