If you want to believe something about someone, first you will convince yourself it is true, then you will act in ways that will make it true. How that person really is, of course, is always more complicated than your limited view of them, but that won't matter to you, and it won't stop you from doing this to them.
We acknowledge complexity in ourselves, but we do not want to acknowledge it in others. We always—and so coincidentally, too!—manage to stumble upon the right point of view ourselves, so anything someone else believes that is different—again, how fortunate for us!—must be wrong.
This inability to accept others as being equally as complex as ourselves, this tendency to consider everything we do and believe to be the right way and all other perspectives to be wrong, keeps us from seeing reality, keeps us from understanding each other as fully as we might, and leaves us stagnant in the world, with no need to learn anything new and, as far as we know, nothing left to learn.
How sad. How dull. How dumb.