The shoulder is a very interesting joint that is designed more for mobility rather than stability. It has a very shallow ball and socket joint, with a strong capsule (collection of ligaments surrounding the joint) to help keep it stable.
Due the great range of movement in the shoulder, that allows us to swim, do handstands and hang out the washing… much of the stability in the shoulder must come from the dynamic control in all of the muscles that surround the joint. These muscles do not need to be very strong in most people, but they must have good coordination to avoid overuse of the many delicate structures in the shoulder complex.
A large contributor to the function of the shoulder is actually the positioning of the upper back (thoracic spine) and the stability of the shoulder blade on the chest wall, so treatment of shoulder pain usually involves correction of habitual postures that put the shoulder in a bad position.