That one is easy....
Humans are essentially congregational beings. We are drawn to each other and are usually found in large groups (ever look at a night-time satellite image of a given county and notice the cities?).
Unfortunately, we are also quite territorial, so being in large groups where another human can impose on your space means confrontation.
Confrontation leads to aggression, which leads to the yearning to succeed over other humans.
Succeeding over others means attaining a position of power and keeping others in line with a set of rules.
We call these rules morals or ethics.
The above behavior can be found in any society, in any city/town/village, and in any household.