Just like in anything that you do, too much can have an opposite effect to your desired results. The same principle applies to exercising.
There have been studies done that show the harmful effects of too much exercise. With this in mind, it is better for professional athletes and casual lovers of exercise to tone it down a bit. There have been biochemical reactions generated by high impact activities done by people who have gone overboard with the amount of training they put in. There is also the possibility of getting hit with an injury because of too much exercise. In this case, the excess physical activity went overboard and beyond what the physical body can handle.
The lure of building a stronger body is there, and that is why people exercise. However, if training is taken further than what the body can handle, it becomes dangerous.
Another source of physical injuries is doing excessive levels of a particular exercise. The problem with doing one exercise over and over again is that it hits the same body part during the excess repetitions. So instead of strengthening that part, it tires due to excessive movement and never fully recovers from continued movement.
If you think about it, most exercise-related injuries are avoided by training and proper gathering of information. The best thing to do is to get the information from an expert. If the expert says to only lift a 125 lbs barbell twice, then do just that.
It is true that people exercise because they want to feel better and have a stronger body. But before doing so, one needs to know his limitations in taking up any sport or exercise to remain safe, too.