Put simply the body is constantly being strained/degenerated with activity and repaired during rest, if you tip the rate of wear to be faster than repair the tissue degenerates until the point it produces symptoms. A little wake up call, will normally allow a reduction in activity and gradual re-introduction and you recover well. However if you rest the tissue (tendon and muscle) degenerates/wastes because of disuse. Sometimes gradual re-introduction alone does not work and we need to use load/training to make the tissue adapt, a bit like a weight trainer making the muscles bigger, although in this case we want to affect the muscle and tendon.