“To have joy, one must share it.”

"Callahan's Law", as expressed in The Callahan Chronicals (1996) [originally published as Callahan and Company (1988)], Part IV : Earth … and Beyond, "Post Toast", p. 388. On the back cover of Callahan's Legacy (1996) this is modified into "Shared pain is lessened; shared joy is increased (and bad puns are appreciated).

The Callahan Chronicals <!-- [Sic] -->(1996) [originally published as Callahan and Company (1988)] "Backword", p. xii
Context: In a culture where pessimism has metastasized like slow carcinoma, that crazy Irishman was backward enough to try to raise hopes, like hothouse flowers. In an era during which even judicious use of alcohol has been increasingly bad-rapped, the man who came to be known as The Mick of Time was backward enough to think that the world can look just that essential tad better when seen through a flask, brightly. (As long as you let someone else drive you home afterward.) Above all, he — and his goofball customers — believed that shared pain is lessened, and shared Joy increased.
Now he is gone. Gone back whence he came, and we are all the poorer for it. But I refuse to say that we will not see his like again. Or his love again.

The Ancestress (Spoken by Bertha, of Jaromir)
The Venetian Bracelet (1829)

Source: Pride and Prejudice

Heaven and Hell #399

Source: The Master and Margarita

Source: Trinity