The way I'm looking at it right now, plenty of human psychology is rooted in assumption.
No matter how often we repeat the possibility of putting ourselves in one another's shoes, we can't literally do such a thing. We can't automatically assume a person's entire repertoire of experiences, mannerisms and habits in a limited amount of time. For that matter, we simply can't do that, period. The most we can do is try to see where the other person is coming from, and try to formulate a sense of empathy with that person based on what we know.
In short, we have to make assumptions about people. We're practically forced to do that, if only because that happens to be one of the easiest ways we can get along.
When we make the right assumptions about a person, then we make the right assumptions about a person. They smile at us and mention how well we've seen the situation through; they warm up to us and place a little more trust in our personality.
But when we make the wrong assumptions about a person, then we do something incredibly horrible. We get accused of misinterpreting things, of not seeing the bigger picture, of running our mouths off and taking things to an illogical conclusion.
It's easy to presume things. It is not easy, however, to maintain consistency in one's presumptions. It's the difference between traversing a short footpath, and then deciding to walk that same path up the side of a mountain: The latter is infinitely -- and at times, fatally -- more difficult.
Ultimately, the problem with these premeditated notions is that we can't help but make them. This is, in a sense, how we associate with other human beings. This is how we live, work, love, and relate. This is how we can maintain good relations with people yet still get into arguments, how we can do so well at work yet still screw up every now and then.
In the end, it is not these assumptions that bring out our personal natures. Instead, we must assume that everyone makes these ill-timed assumptions; We must understand that we're perfectly capable of making the same mistakes ourselves. And when it all comes right down to it, we must forgive.
There is no way by which we can possibly know someone inside and out, right down to the tiniest of cells in their skin. The best that we can do is form a few uneducated conclusions, and hope that they don't offend or insult anyone. What brings us beyond the spectre of mere humanity is our ability to understand how people do this; What makes us more than vessels of illegitimate assumption is our ability to listen to people, and forgive them their sins of impression.
By this time, your eyes have probably glazed over, and you've probably even gone past the point where you regard me as some itinerant writer with a messed-up mind. That's fair enough, I say. I assume many things about the human condition, only some of which manage to make it to the stuff I write.
But I ask you this, then: What assumptions do you form from my words? Do I offend your delicate sensibilities, or do I raise your imaginary hackles? Do you judge this based on what you know of what I write, on who I am, or on what exposure you've had of me?
We can assume all that we want, I suppose. But in the end, we must try to understand.