What is it that strikes a spark of humor from a man? It is the effort to throw off, to fight back the burden of grief that is laid on each one of us. In youth we don't feel it, but as we grow to manhood we find the burden on our shoulders. Humor? It is nature's effort to harmonize conditions. The further the pendulum swings out over woe the further it is bound to swing back over mirth.

Mark Twain: The Complete Interviews (ed. University of Alabama Press, 2006) - ISBN: 9780817315221