The majority of shoulder pain is because of disorder of the rotator cuff.  The tendons of the muscles all attach to the upper arm (humerus). 

Imbalances of the rotator cuff over years of use or trauma tic injury cause weaknesses in the function of the rotator cuff tendons.

Pain is often worse at night when lying on the effected side.  The shoulder will often lock with most difficulty raising arm to the side.  The pain will often radiate down the outside of the arm.