As it is universally known, if a system or a thing has more energy, then it is less stable. If a system has less energy, it would be more stable and would be reaching an equilibrium point.
However, if life reaches an equilibrium point, it just means that the said being is dead!
Because of that, one must eat food and keep gaining energy to remain alive, that is, not reach equilibrium. But too much is too bad. If one gains too much energy and shifts to a complete opposite pole, then that could also lead to death!
This is what failure in evolution does!
When one level up, energy gets accumulated in the body. The base capacity rises each time and after reaching a certain level, one cannot level up unless he/she uses all the accumulated energy in either evolving or in something else.
In this world, evolving is the only task that a beast can do other than increasing its own base stats.