There is a lot of debate amongst shoulder surgeons about exactly why this happens. There is a relatively poor blood supply to the rotator cuff, especially the supraspinatus. Without a good blood supply, the muscle has a hard time healing itself. A spur on the undersurface of the acromion can wear a hole in the rotator cuff, much like a pebble in your shoe will cause a blister on your foot. See the section on Impingement .
Weakness is a very common symptom - especially difficulty lifting the shoulder overhead.
Pain is also a frequent symptom - often patients complain of pain while sleeping on the affected shoulder. Your doctor will perform a physical examination - often this is all that is needed to diagnose the cuff tears in the hands of a physician who has a lot of experience with them. Usually, x-rays will be ordered to look for a bone spur on the undersurface of the acromion. I do not believe that these give any more information than an MRI, hence I do not order them. It depends whether the tear is acute or chronic.