““Will you write the story?”
“If there is one.”
“Happy ending or no?” He was serious.
She attempted a smile. “Fairy tales always have a happy ending.”
He leaned back in his chair. “That depends.”
“On what?”
“On whether you are Rumplestiltskin or the Queen.””

Source: Good in Bed

Source: The Story of Us

Campbell follows with a quote from Ovid's Metamorposes, "All things are changing; nothing dies..."
Chapter 2
The Hero with a Thousand Faces (1949)
Context: The happy ending of the fairy tale, the myth, and the divine comedy of the soul, is to be read, not as a contradiction, but as a transcendence of the universal tragedy of man.... Tragedy is the shattering of the forms and of our attachment to the forms... the two are the terms of a single mythological theme... the down-going and the up-coming (kathados and anodos), which together constitute the totality of the revelation that is life, and which the individual must know and love if he is to be purged (katharsis=purgatorio) of the contagion of sin (disobedience to the divine will) and death (identification with the mortal form).

Source: God’s War (2011), Chapter 39 (p. 285).

Source: Lighthousekeeping (2004), p. 49

Source: Monstrous Regiment

From the published screenplay for "The Big Brass Ring" (Santa Barbara, Calif.: Santa Teresa Press, 1987)
Variant: If you want a happy ending, that depends, of course, on where you stop your story.

Source: Inkspell