Why when a chemical species is stablized by resonance, its energy level is lower? As I know, resonance is only a kind of electrons delocalization within the species. There is no interaction between that species and the environment.
Stabilization is equivalent to lowering the energy. Conceptually, you can think of the charge being more evenly distributed when the electrons are delocalized---thus decreasing the electrostatic repulsion between like-charges, and better saturating between opposite charges (i.e. nuclei and electrons).
These processes don't require any interaction with the environment.