Why do some people who are in relationships cheat?

They want one to fall back on if the other doesn't work out...and they don't want to be the bad person by breaking up for someone else. Cheating is done out of greed, selfishness, and cowardice.

They think that the grass is greener on the other side of the fence. Plus breaking up would mean that they can't admit they they can't stay faithful.

Because they are almost love each other and know each other, so they think chest each other will not hurt each other too