Why do people whistle? Is it because they're happy? Or bored? Are they working out some kind of musical composition in their heads?
Maybe whistling is a kind of unconscious reflex action. Like when we can't sit still, so we drum our fingers and tap our feet. It's similar to when a cat lies motionless, except for his twitching tale.
Perhaps whistling is yet another way for some of us to avoid silence and the contemplation that haunts it. We're just whistling in a grave yard.