Ex boyfriend keeps bringing up the past?

It's been a year since we broke up and he already has a girlfriend. We're still good friends. We talk, we joke, we laugh just like normal friends. However, he still brings up things from our past relationship and says things like "Remember this?" or "This brings back old memories." Why does he do that? He already has a girlfriend and they're doing pretty well. I'm just wondering why he would bring up things that he really shouldn't. I'm currently not dating anyone.