We tend to overestimate our uniqueness [because we] tend to overestimate everyone’s uniqueness — that is, we tend to think of people as more different from one another than they actually are. Let’s face it: All people are similar in some ways and different in others. (..) Because we spend so much time searching for, attending to, thinking about, and remembering these differences, we tend to overestimate their magnitude and frequency, and thus end up thinking of people as more varied than they actually are.
(..)
Our belief in the variability of others and in the uniqueness of the self is especially powerful when it comes to emotion. Because we can feel our own emotions but must infer the emotions of others by watching their faces and listening to their voices, we often have the impression that others don’t experience the same intensity of emotion that we do, which is why we expect others to recognize our feelings even when we can’t recognize theirs.

//Daniel Gilbert, 2007, Stumbling on Happiness