What makes human beings mean to one another?

Selfishness. Pride. The illusion that we are not connected. Lies we are told.

We are all basically animals and whether we want to admit it or not, like other animals we frequently quarrel over mates and territory.

What sets us apart from animals is the extent to which we can be mean to each other.

Animals kill each other, but usually only one at a time.

Humans on the other hand have the ability to annihilate the Earth and every living creature on it, including of course themselves.

Well you should define "mean".

Humans are nasty to others because of their personality, emotions and belief (eg. Hate, Spite, Anger).

From an scientific point of view genes, environmental factors and even evolution (survival of the fittest) can make a differnece

the human being is made of of good and bad, some more than others, some have bad experiences and take it out on others, others jump on the bad wagon rather than consulting their own feelings and others are just plain meanies (hate those meanies)

Medulla oblongata!

we have an emotion called anger, hate, rage.. not that hard to figure out if you thinkk about it.. and ive been drinking

testosterone

we are all different - and confusing