Optimal function of the shoulder joint is dependent on the correct positioning and stability of the shoulder blade (scapula) and the correct positioning of head of the humerus (upper arm bone). The humerus and the scapula move together, this is called scapulo-humoral rhythm. The Scapula needs to be able to rotate upward as the arm goes up to maintain optimal position of the head of the humerus.
Since the shoulder is very complicated, there are many things that can go wrong. Let's look at the one issue.
While lying on your back, reach your arm back, over your head. Optimally, you would be able to lay you arm flat on the ground wothout pain. If you can't or if you have pain, like in the picture above, then you have dysfunction. What is wrong? Commonly, tight muscles are to blame. Which muscles? Well, it could be many including: the pectorals (possibly one downfall of perculating pecs), Lats, Rhomboids, etc. Whichever muscle(s) is (are) tight will require stretching to allow proper scapulo-humoral rhythm.
More issues to come.