Why do some people cheat on others?

I think it's case by case thing. So many factors can influence people to cheat on his/ her partners. Two general factors are internal and external. It's more on phychological thing where people need cannot be fulfilled by his/ her partner. However, I found many cases where there are nothing wrong with the partner, it purely the bad personality of the persons. But most of the cases I think due to the insecurity or absence of emosional need fulfilled by their partner.

Mmmm....I think sometimes it's discontent or maybe something is wrong with his/her partner that he/she can't do something about it but to let it out in a way of cheating...or maybe It's just the animal instict inside us. You know to reproduce! heheheh.