For Kierkegaard, despair is not just sadness; it is a sickness of the self - a mismatch between who we are and who we are willing to be. In The Sickness Unto Death he distinguishes forms of despair, including not wanting to be oneself and, more defiantly, wanting to be someone else. Many people, he thought, live this way without noticing: absorbed in the crowd, borrowing their identity from others, never risking the task of becoming a self. Later existentialists, from Heidegger to Sartre, built on this idea of authenticity. The cure, for Kierkegaard, is the courageous acceptance of the self you are called to become.