Why do people cheat?

I just ended a relationship, & I have a child with this guy. I always had my suspicions that he was cheating. Hell I practically walked in on him & his ex at my house when I was pregnant. Our relationship was going down hill, fast. We were together for almost 3 years. 21 months to be exact. He proposed to me 1 month into our relationship, said I made him feel different. That no one ever made him feel that way & he wanted to spend the rest of his life with me. Well that feeling for him lasted about, 3 weeks before he was talking to his ex. He apologized, he even cried. And things were ok for a little. Needless to say, a month ago I had it, and left abruptly.

Now, he used to tell people I can't wait for her to leave me. I'll be able to party, etc. A friend of his said to him, "You don't want her to leave" and he said oh yes I do. Boy, was his friend right. He is miserable to say the least. He has broken down to me many times telling me he is so sorry for the things that he did to me. That he gets sick thinking about everything he did to me. He admitted to cheating on me with 4 different girls. Before I got pregnant, while I was pregnant & after I had our daughter. He only worked for 1 year while I was with him. I worked, all the way until the week before I had our daughter. And worked after. Like CRAZY. I have forgiven him for what he has done. I have told him numerous times, I will never forget, and he will never get back with me. We are civil for our daughter. We chat, hang out with the baby, etc. He tells me how stupid he was, because if he hadn't done the things he did, we would be married & probably be talking about having another baby right now. And he screwed that up. He tells me he has no idea what he was thinking when he cheated. Now, he hadn't sexually cheated on me in 1 year. But he has cheated on me emotionally, and betrayed me. I was having this conversation with him tonight, about our neighbor who said to him that I told her I had a "fling" with someone at my job. I told him she was full of it. He told me that she had cheated on her boyfriend, whom she has a child with.

He said to me what is wrong with the world today? I told him that something was missing. That spark. The romance. The person who cheats is not happy in their relationship. They are needing something, and their S.O. wasn't giving it to them, so they needed to find it somewhere else. That is why there are habitual cheaters. They cheat, get caught, their S.O.'s don't want to show or give affection, so they stray again.

I think the reason people cheat is, because they are unhappy in their relationship but feel bad about leaving or something is missing that used to be there. My ex says it is because they are unhappy with themselves. What do you think?