Why do I keep having dreams about my ex?

I went through a breakup about a month ago (our second time splitting) and he started ignoring me so I'm pretty much out of his life now, no contact whatsoever. I've been happy though. The one thing that's driving me crazy though is that 2 or 3 times a week, minimum, I have dreams about me and him bickering, over anything at all, stupid stuff. What's weird is right when we very first got together back in july, i started having dreams almost every night of his ex girlfriend coming back and trying to steal him away. for the past month i haven't had those dreams anymore, but i think that's probably because i actually found out that he was talking to his ex again. why am i having dreams now about him? they don't even seem important, just obnoxious. he's out of my life now and it's for the best but how do i get him out of my head? what's going on?