Existentialism is not mournful, no; look at Kierkegaard and Buber. Sartre is beyond mournful — he’s a nihilist. Existentialism pushes people toward painful honesty. Sartre doesn’t push at all. His is an anti-humanism. I know his essay “Existentialism,” and his arguments that his nihilism is a humanism. His arguments are not persuasive. He talked action and did none.