Fantasizing about an ex girlfriend after 10 years

I’ve been having sex dreams and fantasies about my ex girlfriend. I keep thinking about her wearing black bikini to tease me, she has great 38 DD boobs, a voluptuous figure but no oral skills. My current girlfriend and I have a way better sexual connection, she will do anything I want unlike my ex. I just don’t get why I’m starting to think about sex with my ex.