Question

Why is there a reduction reaction every time there is an oxidation reaction?

Answer 1

Oxidation/reduction reactions are always coupled. When a molecule is oxidized, an electron is taken away. But the electron has to go somewhere, it can't just disappear or just float around aimlessly. Therefore it becomes associated with another molecule, reducing that molecule.

Since electrons carry a negative charge then we talk about "reducing" something, what it means is that we are reducing the overall charge of the molecule (for example, from a charge of 0 to a charge of -1).