Why do I keep having dreams about my ex?

I've been having dreams about my ex ever since he broke up with me months ago. our relationship wasn't even that long, and we never got extremely physical or anything. He drives me crazy and I wish I could stop dreaming about him. We don't even talk. So why do I keep having dreams of him if I hate him so much? I understand that dreams are symbolic, and they may have to do with how I'm feeling. I've been down lately, and when I was going out with him, I was as happy as I could be. I haven't really been as happy as that since. Is this why I keep dreaming of him? My mind is trying to tell me to be happy again? Or what?