The shoulder is made up of some very flexible and highly moveable body parts.
The joint itself is actually an open faced platter standing on edge with the
ball end of the upper arm bone (the humerus) rolling around on it. It is NOT
held within a cavity socket like the hip joint is. It's held in place by muscles
and ligaments. The primary ligament is the Rotator Cuff Ligament. It is somewhat
loose and has to allow the bones to move more than any other joint in the
body. To protect this structure there are several strong muscles across this
joint. They are very important because if they don't work properly then whatever
weight is placed on the shoulder is carried only by the ligaments and they
are essentially weak. It only takes a thousandth of a second to tear a ligament
before a muscle responds and tightens up. That can be just as you pick something
up heavier than you expected or do it awkwardly or out of position or move
too fast for a reflexive 
tightening
of the surrounding muscles. It doesn't take heavy weight. In fact, heavy weight,
because we are expecting it and preparing for it, is seldom the cause of the
ligament injury. This should alert you to always take just a slight second
to get ready to use your muscles and joints. It can and usually does happen
other places in the body as well.
In specialized training in rehabilitation of the upper extremity. One of the most interesting concepts taught is that anything connected or affecting the supporting structures of the shoulder can cause pain in the shoulder. If the shoulder itself is treated and not the original underlying cause of the problem then the shoulder will not heal. An example is the underlying ribs which anchor the rhomboid and serratus anterior muscles which pull the shoulder blade forward and backward. If there is something wrong with one of the ribs which causes is to be irritated when pulled on as an anchor, it will cause the muscles attached to it to weaken so as to not pull as forcefully. This in turn will cause the shoulder blade to operate improperly. The next step is the muscles moving the shoulder and arm will have to operate differently in order to compensate. This can lead to the pain in the shoulder which sends the patient to the doctor initially. The original rib problem often will not be checked because the rib problem often isn't bad enough by itself to cause pain. A doctor or therapist can treat the shoulder problem until they are blue in the face and will never correct it until the underlying cause in the rib is found and corrected. The shoulder is often the direct injury but just as often the shoulder is the resultant problem caused by something else. It's important for a good doctor to look beyond the painful area in order to see if something else is causing the pain.
For years doctors and therapist have treated frozen shoulder (the painful inability to move the arm through the normal range of motion) without thinking about the above concepts. Most often it takes many weeks to gain some improvement. Quite often the culprit is spasms in the muscles UNDER the shoulder blade. If these muscles are treated by applying pressure near the spine but going under the scapula to treat the TPs almost immediate relief and improvement can be gained. This can be done by a large hook shaped tool which a patient can pull under their scapula. Otherwise it takes another person to do the therapy and it's somewhat complicated to do it effectively and easily for the therapist.
When doing this therapy it's very important to remember that there are two directions the therapy must be done. First, from the medial edge (near the spine) of the scapula applying pressure along the outside of the ribs but under the scapula itself. It's kind of like trying to create an angel wing out of the bone. In a non-TP person it's possible to push almost all the fingers or thumbs under the bone and it actually feels good. The second direction is from the armpit toward the underside of the lateral edge of the scapula along the ribs and toward the spine. Without this second direction, you're only treating half of the problem.