I think it’s because humor is related to strength. To have a sense of humor is to be strong: to keep one’s sense of humor is to shrug off misfortunes, and to lose one’s sense of humor is to be wounded by them. And so the mark — or at least the prerogative — of strength is not to take oneself too seriously. The confident will often, like swallows, seem to be making fun of the whole process slightly, as Hitchcock does in his films or Bruegel in his paintings (or Shakespeare, for that matter).
