Dreams!

I am currently in a relationship have been with the same guy for about 3 years but I still have dreams about my ex that i was with for about 4 years...it's not all the time but I've had dreams about him for the past 3 nights! I was wondering if someone could tell me why this happens and something that could make them stop... I guess deep down I still love him but that doesn't explain the dreams we have both moved on or he says he has he has 2 children with a girl he doesn't like so I don't understand why I have the dreams