Metals and other substances are made of atoms and every chemical reaction happens when the invisible atoms change their aggregation by sticking to different atoms. This is what results in a visible chemical reaction.
To stick to each other, atoms need to release, share or withdraw electrons of the neighbouring atoms. There are no chemical bonds, no stable unions among atoms, no reactions and either new stable substances if the atoms don't share or exchange their electrons.
Atoms in the elemental state, as the ones that a metal is made of, can only react by losing or releasing electron(s), i.e. by getting oxidized in both cases.
This happens because in the outermost shell, the electrons are less attracted to the atom core in respect to other (nonmetal) atoms. These latter, as oxygen atoms, have a higher attraction for electrons, so they can withdraw electron from metal atoms and form chemical bonds as a result. Oxygen and other nonmetal atoms can oxidize metal atoms.
If you take the same nonmetal oxidant element (e.g. oxygen) and try several metals with it, you'll discover that those metals whose atoms lose their electrons with ease will be more readily oxidized, or at lower temperature. By converse, those metal atoms whose outer electrons are more strongly held, as gold, are very resistant to oxidation.