Why do people cheat on others?

Why do people cheat on others?
I know it depends, but what is the number one reason that everyday young couples face cheating issues?

Lack of proper communication.

This is what I think:
When someone cheats on someone its because they dont love them.
When you love someone they should be the one you think about.
Or they cheat because they are not getting what they want like sex etc.

So heres to answer your question about the number one reason:

The number 1 reason men cheat are surprisingly: emotional dissatisfaction. They are not getting the emotional connection with their wife/girlfriend that they are looking for. In one research 59% of the men said it was emotional dissatisfaction.

Source: http://ezinearticles.com/?4-Signs-He-is-Cheating---The-Number-1-Reason-Why-Men-Cheatid=1517968

probably because they arent getting enough from the first person, but they dont want to hur their feelings and dump them.

I don't know, I think if you even think about cheating you should just break up.

I feel if you even think about cheating on someone you should not be with them, it is disgusting and causes nothing but pain.

Selfishness...someone that cheats is selfish, they are only thinking about what they need instead of about what might hurt the other person.

It usually comes down to an insecurity, or they're just not mature enough to be an adult and break up the them. It's sad really.

temptation is probaly the number 1 reason I think
my 2cents