From a brilliant 2006 column by Gene Weingarten titled The Peekaboo Paradox:
At its heart, laughter is a tool to triumph over fear. As we grow older, our senses of humor become more demanding and refined, but that basic, hard-wired reflex remains. We need it, because life is scary. Nature is heartless, people can be cruel, and death and suffering are inevitable and arbitrary. We learn to tame our terror by laughing at the absurdity of it all.
Speaking of laughter, you simply must visit Laughing at My Nightmare. Shane Burcaw is the nephew/cousin of some good friends and his approach to life is simply amazing.