[aldoxime_amine]

Why should the energy of a system be low? Or better phrased, why does a system tend to attain a low energy state?

Is it the fundamental axiom of thermodynamics? of physics?

My question is not a philosophical one. It is an honest question...

[Hunt]

why does a system tend to attain a low energy state?

As long as you're in the realm of equilibrium states,  each and every spontaneous process under specific constraints leads to a thermodynamic extremum. Under specific conditions such a process leads to a minimization of the energy of the system, but this is not a general statement of the 2nd law of thermodynamics. For example if an object is released from rest at a certain height h , it idealistically falls at const grav acceleration until it hits the ground. Its potential energy has decreased, but the process occured at const V and T. Now take a liquid ( dominated by adhesive forces ) in a capillary tube, due to surface tension and as a function of the contact angle , the liquid rises in the tube. The process here leads to an increase in potential energy yet the principle of thermodynamic extremum is still the same. To say , then, that the energy of a system always decreases is not only vague but wrong too.

For a general statement of spontaneity, only the entropy of the system ( of any type ) for any irreversible spontaneous process reaches a maximum.