It is just as important to develop our as it is to develop our bodies.

Developing you’re self can help you stay out of trouble and making good decisions

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)

Death by starvation.