Why do things interact? Why do things attract? Why do things repel? Physics does not explain why. It can only describe how. The result of all interactions is the force. This force is carried by the quanta of the interactions. If the quanta do not interact with themselves, the physical law can be easy to describe. But when the quanta interact with each other, the physical law becomes intractable and very difficult to formulate. This is the case with the graviton, which are known to interact with themselves. Self-interaction is always difficult even in the well known electromagnetic field theory. For the Higgs field, this field is supposed to interact with itself. but a force cannot be found. Why?