I don't know the answer to why we cling to things, thoughts, feelings, beliefs, hopes, dreams, or people. I do know that we learn this habit of hanging on at a very young age. For many, this habit grows into selfishness. For others, it becomes helps them become misers. Still, for others, it becomes a way of growth. They cling for a while and then realize that the clinging does not serve them, and, they let go.
Animals can teach us a lot about letting go and acceptance. When you free an animal, they are more apt to follow you. When you free them, you treat them as an equal being. You show respect. Perhaps we should think about that in our relationship with humans.