You wear out your shoulder from overdeveloping certain muscles and ignoring the other muscles that support that joint. This applies to every motion that develops muscles.

If you over develop 1 muscle and ignore the muscle that counters it you are going to tear something. The stronger muscle will try to contract as far as it can. If the opposing muscle is not strong enough the bone will try to move out of the socket and you get an injury. It's that simple.

I have bad shoulders from genetics.If I work all of my muscle groups that support my shoulders they feel great. When I ignore certain muscles they hurt.

Shooting my bow at 70#'s doesn't bother them and I can draw back slow if needed while still on target.

Make your body happy and exercise.