Most often, rotator cuff syndrome develops gradually, from aging, continued overuse, repetitive overhead activity, or a combination of these factors.
One of the main reasons that the pain increases when you raise your arm above your head is that the upper tendon in the rotator cuff (supraspinatus tendon) and the subacromial bursa, which lies on top, glide between the underside of the top of your shoulder blade (acromion), and the gleno humeral head [upper end of your arm].
In a normal situation the balance between friction and joint mobility allows for enough room for this tendon and bursa to move and function. But this mechanism will be significantly disturbed when there is not enough room for these structures to do their work and an impingement with a subsequent painful inflammation of the bursa [bursitis] and/or the tendon [tendonitis] will result every time you try to raise your arm.
The major reason for this impingement to happen is muscle imbalance between the rotator cuff and the deltoid muscle due to which the rotator cuff [mainly the supraspinatus muscle] can no longer balance and regulate the upward pull of the deltoid muscle [shoulder] on the gleno humeral head [head of the upper arm bone].
Over time, the tendon may wear against the undersurface of the acromion, causing tiny tears and bleeding. As those tears heal, the original healthy tissue is replaced by scar tissue that is weak, thickened, less flexible, and stringy (fibrous). The gradual scarring of the tendon causes the entire rotator cuff to weaken. The weaker the tendon becomes, the more susceptible it is to partial or complete tears, which happens more in the elderly population.
Without treatment, a simple muscle imbalance can cause a cycle of inflammation, tearing of tendons, and scar formation, resulting in pain and loss of function.