Why do people cheat on each other?

This may be a dumb question, but I really need it to be said out loud. I can only think of people cheating on each other in a relationship as stupid and outrageous, I cannot understand how people can even do such a thing. So, how do they feel when they cheat on someone? Couldn’t they have just ended the relationship first, instead of having an affair? How can some people continuously cheat on people when they know that it’s hurting the other person?