According to existential philosophical theory, we all live lives of quiet desperation. Some aspect of our existence is limited in such a manner that each and every one of us experiences great pain or frustration. An existentialist believes the way to achieve happiness is to learn to accept and/or love your limitations.
Sisyphus is a figure in Greek mythology who scorned the gods and hated death. His disobedience earned him a terrible punishment: he was sentenced to spend eternity in back-breaking labor that accomplished nothing.
He was ordered to spend all day every day pushing a gigantic boulder to the top of a steep hill only to have it roll back where he started. For Sisyphus to find happiness from the existentialist perspective, he must learn to love his rock.