Also from Hagakure (though I know it from Ghost Dog: The Way of the Samurai):
Among the maxims on Lord Naoshige’s wall, there was this one: “Matters of great concern should be treated lightly.” Master Ittei commented, “Matters of small concern should be treated seriously.”
Reminds me of a famous quote from Danish humorist Storm P:
“He who understands humor as only humor and seriousness as only seriousness has misunderstood both”
Also from Hagakure (though I know it from Ghost Dog: The Way of the Samurai):