It seems like a lot of people here are talking as though cheating means they want to trade their partner for the person they're cheating with. Sometimes, sure.
But nah, I think more commonly actually people just want to have their cake and eat it. Knowing it's wrong but lapping up the pleasure. They enjoy being with their partners and they enjoy the excitement of a fling, so they do both hoping the partner simply won't find out. You can dress it up and try to make up other reasons in these circumstances, because it's hard to admit that you would risk hurting your partner so badly for fun - really though, it is just for fun. I have met several married men who live by the idea that "what she doesn't know won't hurt her", one said 'it makes the relationship better, keeps it stronger - you are refreshed'. They're not hypocrits though, they insist that what they don't know won't hurt them either! When one married he said to his wife the next day 'You can do whatever you want, I just don't want to hear about it'. Although mostly people don't say anything like that, they just do.
I have never cheated and never will. Personally that kind of lifestyle is not for me at all! Genuine mutual love, respect and trust all tha way. I couldn't stand the idea of touching anyone else or seeking pleasure and gratification from anyone other than my partner. We're a unit.