Do happy people cheat?

Yes. I used to be in a really happy relationship. We could talk from one topic to another without feeling bored, we could mess around with each other and cuddle and watch TV before sleeping. We could share everything and anything with one another. Then one night.. I woke up and saw him on his mobile, in a chatroom looking for sex. There is no definite answer why people cheat. Excuses, plenty. After a long time, I realized it was simply bcoz he didn't love me enough to NOT hurt me.