The most primitive foundation of the mind is ego. Its primary mandate is survival by any means.possible, and in that pursuit one seeks to gain advantage at every turn, which naturally leads to conflict.

Moreover, as one gains advantage one somehow rationalizes it to be fair, but when others gain advantage it seems brutally unfair because one instinctively feels one's survival threatened

What offsets pure ego is intellect. That is where society makes up rules and regulations to try and mediate fairness.

Donald Trump is a prime example of a nearly perfect primitive ego. There is little intellect to round off the edges of his raw ego. Thus in Trump's world anything that does not give him enormous advantage he sees as being grossly unfair. Thus he is always exhibiting childish tantrums as he feels his survival constantly threatened.