1With increase in energy a compound becomes less stable. And with loss of energy in a reaction the product formed becomes more stable.
I would just give you an example because this is something one must necessarily understand. Take a stone and throw it upward as it reaches certain height why does it come back because it has undergone a change in potential energy. Whenever there is an increase in potential energy the stability decreases.
Thats why among Resonating structures the one with least energy is more stable. That is why bond formation is exothermic reaction. And thats why when we separate two unlike charges their potential energy increases.