Rosh Hashana's solemn role of affirming that G-d judges us, makes its central theme of laughter difficult to understand. Regular worshippers are accustomed to finding the meaning of the day in the Torah portion designated for public reading on that holiday. Even from conception, laughter surrounds his life. With the meaning of Isaac's name as well as the laughing sounds of the day all integrated by the day's reading of the Torah portion, Rosh Hashana is not only the day of judgment, it is also the day of laughter. Laughter is one of the distinctions that humans enjoy over animals.
People laugh at things that violate a sense of how things ought to be.
A pompous mayor who slips on a banana peel is funny. A tramp who falters and sprawls on the sidewalk just seems sad. Likewise, the sexual innuendo that provokes howls of laughter among school boys and titters among stockbrokers, elicits yawns of indifference from hardened prison inmates. The joke assaults notions of human refinement, thereby causing laughter. To the depraved it is not a joke, it is reality.