Dreaming of ex's

Ok I'm not sure why, considering I'm engaged now and to this day have no connections to this person anymore, but why is it when you've moved on years after an ex(specifically my first love and oldest child's father), you find yourself dreaming about them? Our relationship ended in 2009 because he cheated and I returned the favor. He married and I went on to my next heartbreak relationship and now I'm very happily engaged. I woke up this morning after finding myself dreaming of him and his son from his marriage helping me and my child run away to Mexico and we end up sparking an old flame. WTH!!!!!! I havent even talked to him in two years so why all of a sudden am I dreaming about him?