I love the comic Subnormality by Winston Rowntree. Particularly poignant to me is one entitled, We Assume of Others What We Know of Ourselves. The point is right there in the title and the examples are in the comic. I have often directed people to that comic or otherwise imparted that lesson to them ever since I came across it.
I am not going to go on about that subject because it is so perfectly explained in the source material. What I am going to talk about is why it's true.
How come we assume of others what we know of ourselves? Why would we think that everyone plays by the same rules?
Well, first off, many people only know one set of rules. We live with ourselves. We are constantly in our own head. We spend every waking minute (and many sleeping ones) thinking about our surroundings and doing so in our perspective. When you become so saturated, it becomes a struggle to see the world in any other way. When you ever see somebody "acting strangely", you don't think that they are playing by different rules; you think that they are playing by your rules, but breaking them.
This can be a curse to writers, only seeing the world in one way. This is why I am always trying to get into people's heads, trying to figure out their rulebooks, trying to find out what they assume of others. I need to know as many frames of reference as possible. If not, I will simply have a cast of characters which are carbon copies of me. And that is a one-way ticket to Snooze City.