The shoulder is unique in the body in that it is a floating system on our trunk. Muscles hold everything in place and should precisely guide the shoulder complex whose bones consist of the shoulder blade (scapula) and the arm bone (humerus). The scapula is supposed to rest and move in a fairly precise way. It is where the shoulder socket is found.
Typically shoulder pain is due to the scapula not moving well, which then causes the humerus to move excessively. This stresses the shoulder joint as well as the muscles controlling the humerus.
Also a common problem is overdevelopment of a certain group of muscles that rotate the humerus inward. Excessive or continued rotation such as this, causes tissues to break down.
Layered on top of all of this is whether you are generally a tight person or a loose person. These present twists for shoulder rehabilitation because it potentially changes the stress the shoulder joint tissues are dealing with.
For instance a tight person may have compression acting on these tissues where the loose person has excessive mobility in the shoulder joint. Two completely different stresses to consider.
The last, and perhaps biggest, determinant of your shoulder pain is how you use your arm and shoulder. You may have developed movement habits that are continually stressing shoulder joint tissues.
Your tissue (whether it is a muscle strain or tear, tendinitis, bursitis, arthritis, or ligament strain or tear) is becoming injured because of excessive force being applied to it over prolonged periods of time; because these harmful movement habits haven’t been identified. In other words, you are experiencing overuse problems.
Overuse can happen even if you don’t do the activity for months or years. Because it’s the subtle uses of your arms that are keeping the tissues stressed and just under your pain radar.