[Bartweiss]

By six, you're usually at something like "because that makes me happy". By seven, you're into metaphysics and cognitive science.

Of course, some issues are genuinely simple, and you're off in unhelpful territory halfway through. I suspect five why's is a decent estimate, but not necessarily more accurate than four.

[joezydeco]

There's a really good Louis CK routine about this, involving his young daughter.