Why do I keep having dreams about my ex-boyfriend and it's been 4 years since we broke up?

It's been four years since my ex-boyfriend and I broke up. The reason why we broke up is because he ended up cheating on me while he was away to college. At first I was livid but eventually I had forgiven him and move past the cheating. About two years later he called me out of nowhere and said he was really sad because his current girlfriend had done the same to him. He wanted to know if I would come down to see him and just hang out. I went down there and hung out with him and I started having the same feelings all over again. Before I left we starting making out but that was it. I eventually drove back home that night and never heard from him again. He told his mom that I was too pushy and that he didn't like me like that. It turned out that he was using me in some sorts. When I had gone down there he took me to a restaurant that his ex-gf worked. At the time I didn't know and still don't know if she was working that day and he was only trying to make her jealous. Anyways, I have been dreaming about him ever since then. I have had many relationships after him and am currently in one now and we are expecting a baby. I feel bad when I dream these dreams because I'm in love with my current boyfriend but I always have a nagging feeling on why it is that I keep having dreams about my ex. What could this possibly mean?