Still having dreams of my ex

So I’ve been in my current relationship for 5 yrs and I have a child and another on the way. But for some reason I keep having dreams about my ex and I must admit that the chemistry between me and my current bf is not the same as it was with my ex. But it’s been almost 9 years😩 since me and my ex broke up and I don’t understand why do I keep having dreams about him. We was teenagers when we was messing with each other and I’m now about to be in a few more years. Why is he still stuck in my head ? I’ve been in 2 relationships within these 9yrs since me and him broke up why am I still dreaming about him??