Good evening. I have been wondering what causes quantum systems (or systems in general) to drop down to the lowest energy state. A hydrogen atom absorbs a photon, its electron "jumps" to a higher energy level, and that excited state lasts for only an instant before the atom emits the photon to return to the ground state. Why don't atoms stay excited? Why is there this tendency of systems to always try to occupy the lowest energy state? What causes this mechanism? Thank you very much in advance.