The emotion of ego, considering himself superior and others inferior is such a big problem that exists in the human race. Anyone having such emotions can never collaborate with others conveniently. Their ego resists them.
To reckon how massive this problem is, we need to go back in past. Remember the most notorious characters of WW2, Adolf Hitler, and Benito Mussolini. Their thoughts about their rivals were astonishing. There is an extract of a conversation between Hitler and Josef Hell in 1922: If I ever really in power, the destruction of the Jews will be my first and most important job – Hitler. And what he had done with them is difficult to imagine and ended up the wage of WW2.
Another instance is of Britishers. All the time they ruled their colonies, they always treated their people like mere a device that can breathe. For the people of colonies were just the subject of slavery, humiliations, and not more than any machinery working for them.
History is full of conflicts/wars which broke out just because of the egoistic behavior of some personalities.
The very same is the reason behind criminal actions such as honor killings, and acid attacks. It becomes so difficult for anyone to accept the refusal of his proposal. Either they are parents or any lunatic lover, they see it as their insult. And that is where they make mistake.