Why must oxidation be accompanied by a reduction?
By definition, oxidation is the LOSS of electrons; whereas reduction is the GAIN of electrons.
So, since matter and charge are always conserved, any loss of electrons from something, must result in a transfer of electrons to somewhere. Typically, it is the oxygen atom that accepts the electrons; hence electron transfer to oxygen is oxidation. Can you assign oxidation numbers to the following combustion?
What is reduced, and what is oxidized?