People cheat for the same reason that people lie and cover up their tracks in every aspect of their lives; because they fear the consequences of their actions to those affected around them more than they are willing to admit that they are enjoying the act that they are perpetrating.

One guy's lie is another one's right to "live their lives" depending on your angle and involvement. Whenever someone is cheated there will be pain and suffering. If you're the guilty part then you give a rat's butt because you're having fun and to hell with anyone else having feelings; if you're the cheated one, then you can't fathom why the other person wasn't just clear from the beginning and couldn't tell the truth and save the suffering.

Human beings that don't cheat or lie through their lives are called nice people and good things happen to them.