“It’s an odd thing, happiness. Some people take happiness from gold. Or black pearls. And some of us, far more fortunate, take their happiness from periwinkles.”

Source: The Kite Runner

Relling, Act V
The Wild Duck (1884)

Source: The Wonderful Wizard of Oz (1900)
Context: "All the same," said the Scarecrow, "I shall ask for brains instead of a heart; for a fool would not know what to do with a heart if he had one."
"I shall take the heart," returned the Tin Woodman; "for brains do not make one happy, and happiness is the best thing in the world."

"The Art of Living", interview with journalist Gordon Young first published in 1960
Variant: [T]here are as many nights as days, and the one is just as long as the other in the year's course. Even a happy life cannot be without a measure of darkness, and the word "happy" would lose its meaning if it were not balanced by sadness.

Source: Much Ado About Nothing

Source: The Unbearable Lightness of Being

Letter to his mother, written from the University of Pennsylvania (12 February 1904), published in The Selected Letters of William Carlos Williams (1957) edited by John C. Thirlwall, p. 5
General sources

Source: Wicked Deeds on a Winter's Night

Source: Confessions Of Max Tivoli