“I laugh because I must not cry, that is all, that is all.”

Also sometimes attributed to Kurt Cobain
Variant: You laugh at me because I'm different, I laugh at you because you're all the same.

"Entertainment or Education? (1936)
Context: The theater-goer in conventional dramatic theater says: Yes, I've felt that way, too. That's the way I am. That's life. That's the way it will always be. The suffering of this or that person grips me because there is no escape for him. That's great art — Everything is self-evident. I am made to cry with those who cry, and laugh with those who laugh. But the theater-goer in the epic theater says: I would never have thought that. You can't do that. That's very strange, practically unbelievable. That has to stop. The suffering of this or that person grips me because there is an escape for him. That's great art — nothing is self-evident. I am made to laugh about those who cry, and cry about those who laugh.

Vol. 2, p. 231; Letter 92.
Clarissa (1747–1748)

"Entertainment or Education? (1936)
Context: The theater-goer in conventional dramatic theater says: Yes, I've felt that way, too. That's the way I am. That's life. That's the way it will always be. The suffering of this or that person grips me because there is no escape for him. That's great art — Everything is self-evident. I am made to cry with those who cry, and laugh with those who laugh. But the theater-goer in the epic theater says: I would never have thought that. You can't do that. That's very strange, practically unbelievable. That has to stop. The suffering of this or that person grips me because there is an escape for him. That's great art — nothing is self-evident. I am made to laugh about those who cry, and cry about those who laugh.

Source: The Piper's Son