Why do i still always think about my ex after so long of being broken up?

so i met this guy jorge on face (dont worry he goes to my school) and we started talking and he ended up asking me out ( on valentines day) he also took me to a concert but i still think about him allot nd even listen to some of our songs but we only went out for like 3 weeks nd i miss him but i know he has moved on nd he has a new girlfriend nd he is best friends with my guy best friend.