Why when we know we deserve better, don't we do better?

Why do we settle? Why do we chase someone and yearn for someone who doesn't want us in the same capacity as we want them?

There is a guy.. I may be in love. He isn't bad to me but he isn't good to me either. He is emotionally unavailable, I know I deserve better but I can't seem to let go.

Why is it so hard to let go, when we know we want and deserve better?