"Hell is other people"

We all know hellish people, or people we'd like to send there (or at least threaten to, if there was a there there).

But Sartre's point was that it's damnably de-humanizing to be objectified by people who will not acknowledge your subjectivity-the core of your humanity. This is not the misanthropic statement it's often mistaken to be. It's a call to recognize the inner lives and the human Being in others that true misanthropes disregard. "Sartre argues that consciousness is radically free , never fixed, always choosing itself. Other people threaten that freedom through what he called the look: their gaze objectifies, turning you into something static and defined. In No Exit, three characters cannot escape each other’s looks after death, and that permanent objectification is the hell he had in mind."

William James said it better, and said it first:

...Now the blindness in human beings, of which this discourse will treat, is the blindness with which we all are afflicted in regard to the feelings of creatures and people different from ourselves. We are practical beings, each of us with limited functions and duties to perform. Each is bound to feel intensely the importance of his own duties and the significance of the situations that call these forth. But this feeling is in each of us a vital secret, for sympathy with which we vainly look to others. The others are too much absorbed in their own vital secrets to take an interest in ours. Hence the stupidity and injustice of our opinions, so far as they deal with the significance of alien lives. Hence the falsity of our judgments, so far as they presume to decide in an absolute way on the value of other persons' conditions or ideals.

Take our dogs and ourselves, connected as we are by a tie more intimate than most ties in this world; and yet, outside of that tie of friendly fondness, how insensible, each of us, to all that makes life significant for the other!--we to the rapture of bones under hedges, or smells of trees and lamp-posts, they to the delights of literature and art. As you sit reading the most moving romance you ever fell upon, what sort of a judge is your fox-terrier of your behavior? With all his good will toward you, the nature of your conduct is absolutely excluded from his comprehension. To sit there like a senseless statue, when you might be taking him to walk and throwing sticks for him to catch! What queer disease is this that comes over you every day, of holding things and staring at them like that for hours together, paralyzed of motion and vacant of all conscious life?
...
On a Certain Blindness in Human Beings (1899)