Maybe it just the way I was raised, but I've never understood the need of many people to associate only with people who are like themselves in some way. And further still, to take pride in this collective identity. Can someone explain this phenomenon to me?
??? What is there to explain? Is it really that mystifying? Humans associate most closely with humans, even though some humans seem to connect better with cats and dogs than with other humans. Some cats "run with" with other cats, dogs with dogs (although domesticated dogs, unlike wild ones, will associate with anything that'll play with them), starlings with starlings, garter snakes huddle together in holes in the ground with other garter snakes, and so on.
If you are talking within a species, well that's a different matter. Humans are the only non-predatory creatures I know of that are finicky about others of the same species at a core level. If one is to believe the idea that humans have souls and other animals don't, it'd have to be the soul that explains the difference. If human intelligence is what one accepts as the defining distinction between humans and other animals, one would have to argue that intelligence is the reason.
I think we both can see just how ridiculous either of those propositions is, and therein lies a huge part of the problem: a great many humans espouse principles that really don't hold up under close scrutiny. Humans have a knack for hubris which allows for individuals believing whatever the heck they want to and behaving however they want to on the basis that they "think" they make some damn sense. Of course, thinking is the one thing such individuals didn't do very well in arriving at their conclusions, unless they thought enough to figure out some cockamamey justification for whatever the heck they happen to think.
So the phenomenon about which you ask is caused by one of a few things:
- Ignorance -- this is the cause when one just doesn't know enough to arrive at a valid conclusion.
- Willful ignorance -- this is the cause when one knows enough to arrive at a valid conclusion, but one refuses to apply that awareness or seek the required information.
- Stupidity/dumbness -- this is the cause when one has the data available, is familiar with it, and just can make sense of it no matter how hard they try. Few folks who haven't legitimate physiological mental deficiencies are actually stupid.
As you can see from the above, willful ignorance is the most likely cause one will encounter. Why might one be willfully ignorant? Well, avarice, pride, wrath, lust, envy and sloth/acedia, individually or in concert with one another, are the prime motivators for that.
How does one overcome those failings? Actually, it's not all that hard to do: without exception, treat others as one would have them treat oneself. When everyone does that, all will be well with the world.