To answer your question, its because we don't have to like everyone in the world.
I know that seems simplistic, but we grow up with some fundamental intrinsic thoughts.
1) That the fit survive, so therefore, who ever is in power is therefore the fittest. If you are not in the power chain, than you resent those that are. And if you are at the top, you can't sympathise with those that don't take the steps to improve their situation.
**Now the problem with that lies in that what does one consider powerful or less powerful?. How do you measure that? In a way, there are multiple levels in which people assert their power and give up their power. There is the basic family group in which for example, the older brother might be resented by the younger brother for all that he has to gain. In a larger level, things like religion (with sayings like "our religion is the true religion"), culture "our culture is the strongest culture", regions "our country is a leader, your's isn't"...and so on down the road.
2) Our values, norms and societies define who we are. So therefore how we get along in society depends on where we percieve ourselves in society. And what cards we are dealth.
So my point is that in theory, it sounds good to just get along with everyone. But how we grow up shapes our views and the silent interchange of power that is exchanged in society every day.
Unless we are all the same culture, relgion, gender, socio-economic class with the same education and income, there is no possible way that we can all be judged on an equal footing. Therefore, there will be those that are at the top of the percieved heap, and those at the bottom. And how you percieve your level is based on your own values.
3) Borders, relgion, money, politics. We will never agree on that level.