Why is it so hard to let go?

Old friendships that have lost their charm and even trust. There are constant misunderstandings. All that's left is envy and jealousy. Every word and action is judged bitterly. We know we will never belong in that group. We have run out of reasons to try yet again to be a part, to forget and forgive and start once again. Still why is it so hard to let go and move on? Why do we still wonder whats happening there? Why do we still talk about them so often? Why does it seem so unfair that we were never accepted for what we were? So unfair that all our effort was futile. Why do we still care when we don't want to? Which one of them did this to us and why? It doesn't really matter but why does it still bothers us?