Right, so when you imagine yourself, you think of yourself as a massively complex individual. You may hate yourself or like yourself or whatever, but you certainly think of yourself as fully human. As Whitman puts it, “I contain multitudes.”
The problem is that your brain is the only brain you’ll ever have; your eyes are the only eyes you’ll ever see out of; your experiences are the only experiences you’ll ever know as your own. This is what makes it so easy to dehumanize people—to say, for instance, as Aristotle famously did, that some people are just naturally born to be slaves. But it also makes it easy to dehumanize people in subtler ways. (I’d argue, for instance, that I am able to spend $90 a month on cable television while 2 billion people live on less than $60 a month only because I do not feel those people’s joy and pain and desire as acutely as I feel my own. If I did feel every individual’s need as acutely as I feel my own, I would almost certainly forego cable TV and send that money to those who need it for food and shelter.) But in addition to dehumanizing people, we can also imagine them as more than human: When we think of celebrities, or those we love romantically, we may see this as superhumanly free from the fear and pain and despair that plague the rest of us.
So anyway the task of understanding the reality of other people’s experience is incredibly difficult, because you are stuck being you, and can never even for one second be them. But this is true not only for people who live very different lives from yours, but also for those closest to you. You see everybody in your life in the context of you: YOUR sister, YOUR best friend, YOUR mom, YOUR nemesis, whatever. But they do not see themselves that way. They see themselves as the center of history, just as you see yourself.
This turns out to be a really big problem that (at least in my experience) can only be solved by empathy, an imperfect and incomplete tool (see my $90 monthly cable bill) but the best one we have.