Why do people cheat?

I am not about to answer that question because I have no idea why. I have a friend who is what you call perfect physically, per the men out there and she has a really good personality too. She has dated men and they have cheated. For some odd reason she tried dating women and they still cheated.

Many may think that there must be something wrong with her that is why people cheat but yet when I relate this to my own relationships I have noticed that a number of them have cheated on me too. Now cheating was never my thing that is why I never chose to even try it. Why can't people be mature enough and trust the person that they once loved will appreciate it more if were told sooner about the situation? Why would they lag and let time pass them by then before you know it you have wasted a year or two fooling each other? It is bad enough that they cheated on you but to even realize the time and effort wasted makes it even worst.