Oppression

Why do some people oppress others? There is a lot of anger in the world; lately, I have come to believe that much of this anger is a result of oppression. Oppression begets frustration, which begets rage. What causes some people to oppress others? Is this a natural inclination? Do certain animals oppress others in the wild? Is there oppression within animal species, such as there is with humans? Is the act of oppressing another always a conscious choice, or is it somethings that is done subconsciously at times?