Oxidation-reduction reactions occur because of the competition between particles for

            Oxidation-reduction reactions occur because of the competition between particles for Electrons.

Explanation:
                   Oxidation is defined as the loss of electrons. While, Reduction is defined as the gain of electrons. We also know that oxidation and reduction reaction happens simultaneously. Therefore, the electrons lost by one element are gained by another element.

Example:
                                  2 Fe  +  O₂     →      2 FeO

In above reaction the competition for electrons is found between Oxygen and Iron. 

Oxidation of Iron:
             
                                     2  Fe     →     2 Fe²⁺  +  4 e⁻

Reduction of Oxygen:

                                     O₂  +  4 e⁻     →     2 O²⁻

Hence, in above reaction oxygen wins the competition by gaining the electrons from Iron.