Even the best things for our body can be dangerous when taken to the extreme. Drink too much water and our cells become dangerously diluted. Inhale too much oxygen and our body becomes toxic.. Eat too much cabbage and.... well.... its probably not good. High intensity exercise is no different.
Too little and our body suffers from poor physical fitness. Too much and our body suffers from fatigue, pain and injury. Despite the problems found at either end of the spectrum many of us tend to leapfrog from one extreme to the other when really, like so many other things in life, we would be better served by seeking out the sweet spot in the middle.