Hell is other people. [From No Exit (1944)]
In French, this phrase reads L'enfer, c'est les autres (also translated as "Hell is the Other").
Do you ever get bugged when other people look at you? Like, maybe they are judging you? Well, then you and I have something in common. I'm not a fan of the idea of being a seen object, and so I made this little declaration in my famous play No Exit.
The three characters in this play have a problem with being seen and judged by others and get pretty adamant that they are the only ones with the right to judge themselves. Well, I hate to break it to these peeps, but the fact that other people exist at all means that they will always be seen and judged. And that's a bummer, or "hell," as I put it.
Or, to put it another way (not to quote myself or anything), "When we think about ourselves, when we try to know ourselves, […] we use the knowledge of us which other people already have" (source). Basically, try not looking in the mirror for a week—well, then you're on your way to becoming an existentialist.
Three o'clock is always too late or too early for anything you want to do. [From Nausea, 1938]
Let's not read too much into this one. We've all felt this way a time or two. It's too late to take a nap, too late to have a double espresso, and too early to go to dinner. Three o'clock is the dead zone of the day. Call it a rare moment of folksy insight from me. And people say I'm inscrutable and high falutin'.
In order to make myself recognized by the Other, I must risk my own life. To risk one's life, in fact, is to reveal oneself as not-bound to the objective form or to any determined existence—as not-bound to life. [From Being and Nothingness, 1943]
Okay, get ready for me to drop some knowledge on you: you and you alone are in control of your own life, which is good, but that also means that you have to stand up for yourself in order not to be treated as an object.
See? Your goal is to be a subject, not an object (like, say, a pair of flip-flops or a Snickers wrapper). Only when you put yourself out there in the world (by taking risks) can you be a subject—a "being"—that is not made or controlled by others. "The Other" is that pesky human (or, rather all humans who are not you) who is trying to be a subject at your expense. AVOID THIS AT ALL COSTS.
In a word, man must create his own essence: it is in throwing himself into the world, suffering there, struggling there, that he gradually defines himself. [From Characterizations of Existentialism (1944)]
So let's review: you create who you are, you aren't born who you are (it's a nature vs. nurture thing). Since it's not all rainbows and unicorns out there, you're going to make some mistakes, suffer, be rejected, feel the pain. But only by accepting that life will throw a few obstacles your way can you become who you are.
Step by step, through good and bad, you will become "You," a fully "realized" subject. But you have to fight the good fight at all times. You never actually arrive at your essence—you're always working your way there. I know it sounds like a self-help book, but that's how I roll.
I respect orders but I respect myself too and I do not obey foolish rules made especially to humiliate me. [From Dirty Hands, 1948]
This little gem was said from one character in this play to another, but it gives you a good idea of my idea of authority and "the Other." It's one thing to signal when you're turning left and say "please" and "thank you"; it's another thing to follow rules that make you look (and feel) like a jerk and dehumanize you.