Camus opens The Myth of Sisyphus with the only question he thought mattered: if life has no built-in meaning, why not quit?
His answer arrives through a myth. Sisyphus is condemned by the gods to roll a boulder up a hill forever, and it rolls back every time — pure, pointless, eternal labour, and a fair picture of a life with nothing underneath it.
Then Camus does something strange. He puts the camera on the walk back down.
That descent is where he locates the freedom. No boulder, no illusion, fully conscious of the absurdity, and still uncrushed by it. The gods wanted punishment and got a man who declines to be miserable about it.
The last line is among the most quoted in philosophy and among the most misread — one must imagine Sisyphus happy. His happiness rests on the climb having failed to defeat him, and on nothing else.