I dont believe Humans can get along because of one thing: "The Pursuit of Happiness".
Sounds abstract but if you think about it, its really not. I hate to jump straight to politics, but consider this;
Politician makes a decision that
All people with Natural Brown Hair are now exempt from Taxes.
Now People are split into two categories. Those with Brown hair and those without. Those without are not directly hurt or affected.... yet they are now pissed off because they didn't get in on the tax break. Brunettes are happy because they got the recognition they've always wanted and they don't understand why everyone else is upset... which means they must be jealous right?
Animosity between the two groups escalates for no reason other than the pursuit of happiness. Its perceived that Brunettes are now Happier than all others... and the natural reaction is mostly based in Jealousy, which spawns anger, which leads to hatred, which has potential for Violence.
As ridiculous as that scenario is, I honestly believe thats the cause of all problems. People are so concerned with their own Pursuit of happiness that the easiest way to judge it is by doing so against other people. And if other people are happier than you... you become frustrated, question why, question why do they deserve that excess happiness and you dont?
I believe this is a large reason why there is such disdain for the upper class. Sure they are in positions of power and have pockets lined with more cash than whats in my life savings... but why should I be angry at them? Why couldnt we get along?
To further the Stigmas, the anticipation of frustration becomes the norm. The Brunettes then see Blondes and immedietely think "They dont like me" which completes the circle as the inability to get along is cemented in. Even if said Blonde had no issue with the Brunette in the first place, they'll then experience the anticipation of frustration and get the impression "Brunettes are stuck up assholes".
What has initially caused the animosity that will never allow us to just get along? In my honest opinion, I believe alot of it is based in Religion. To me Organized Religion has spawned some of the worst bloodbaths in human history. If not directly, then without a doubt indirectly. People are taught that their religion is superior and all others will not be admitted into their Heaven, which means they either sit in a Purgatory or even Hell itself... all for not being a Customer of that church. But yet the other side looks at that scenario the same, only from their perspective. Those who are not followers will not get into the Paradise, those who have been passionate & prolific followers will be Kings among men.
I've yet to talk to a devout Catholic that would disagree I'm destined to burn in hell, or an Adamant Christian that thinks I'm worthy of their Heaven, or a Steadfast Muslim that doesn't think I'm the Great Satan already. What bigger hoax designed to pit human against human in our "civilized" history can you think of than that?