Why do ex boyfriends find it hard to be friends?

This is going to be weird, but there was this and the City episode where Carrie says "When the relationship ends, where does all the love go?" and the fact of the matter is, the love is still there...so if you're friends with someone you still love, when they move on, that's going to hurt. You don't want to see someone you once loved loving and being loved by someone else, no matter how much you think you're over them.