In case of religion, this is more or less natural state of mind. After all, I'd struggle to think of a religion that didn't claim to be the one and only answer to life, universe, and everything (hint: we already know, it's 42). So, I won't discuss religion as a cause here. I may do it some other time, though. We, religion and I have an axe to grind.
In fact, what I think I noticed as pretty much a rule, is the same rule tends to drive the "choice" of religion, too. Quotes around the "choice" are there for quite the same reason, too. It seems to me that early upbringing is key here. And by "early upbringing" I mean the one imposed on us by our immediate family at a very early age, probably no later than the end of primary school, in fat. In my experience, regardless of the type of society, or the schooling system, most people tend to at least strongly lean towards what their parents believe(d). If you look closely enough it can also be seen in the people who have ostensibly broken away from their familial traditions. Even in their new value systems they tend to lean towards what they learned at home. And, of course, that includes how they see other people.
For an average person, I would estimate that upbringing is responsible for at least two thirds of their world view, another quarter being the result of the system/schooling. This leaves just less than one tenth to be modified by whatever other influences there may be, persons own free will included. I know this sounds like a horrible exaggeration, and for a prominent few it probably is. However, I am here talking about masses and averages. Of course there are people able to drag themselves away from the mold. If there weren't, we'd still be hunter-gatherers most likely!
And to think that, all prejudice removed, we're all pretty much exactly the same!
So, how do we change this? I think we already are, but the process is painfully slow. There are places in the world now where the beachhead of "equality" (for want of a better word) has grown so much as to become a seaside fortress. I am deliberately not naming any names. There is no need. Even the ones who still think they're better than some others know where these places are, and they hate them with a passion. Also, there is a danger of these places starting to think too much of themselves, and turn into the other kind through seeing their less "advanced" brethren as somehow less worthy. What a terrible crime that would be!
Therefore, we need to tread very carefully. Here, speed is indeed the device of Satan!
But before we move on, may I offer a quick starter lesson here? Yes? Thanks!
Look at the photo above. Now tell me what do you see.