Why Shoulders Easily Dislocate

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The shoulder joint is designed for total freedom of movement, rather than stability.  Thus, the glenoid cavity (the socket into which the head of the humerus fits) is shallow, and the humerus head fits like a golf ball sitting on a tee.  Such an arrangement is far less stable but far more mobile than the hip joint in which the head of the femur fits into the very deep acetabulum for stability and weight-bearing.  Becuse of the shallowness of the glenoid cavity, and because the shoulder's anterior and inferior reinforcements are weak, shoulder dislocations are quite common, especially for baseball pitchers.