Why does oxidation always occur when another substance is reduced?

Oxidation and reduction are coupled processes that always occur together in a chemical reaction. This means that when one substance is reduced, another substance must be oxidized, and vice versa.12

Reduction is defined as an increase in the number of electrons associated with an atom or group of atoms. The electrons that are gained by the substance being reduced are supplied by another substance, which is consequently oxidized. Oxidation, in turn, is a chemical process where an atom, ion, or molecule loses electrons.23

This reciprocal relationship is fundamental to all oxidation-reduction (redox) reactions. For example, a substance that oxidizes another substance is called an oxidizing agent, and it does so by accepting electrons. Conversely, a reducing agent is a substance that reduces a chemical compound, typically by donating electrons.145