“There was never yet an uninteresting life. Such a thing is an impossibility. Inside of the dullest exterior there is a drama, a comedy, and a tragedy.”

Letter to Anne, Countess of Ossory, (16 August 1776)
A favourite saying of Walpole's, it is repeated in other of his letters, and might be derived from a similar statement attributed to Jean de La Bruyère, though unsourced: "Life is a tragedy for those who feel, and a comedy for those who think". An earlier form occurs in another published letter:
I have often said, and oftener think, that this world is a comedy to those that think, a tragedy to those that feel — a solution of why Democritus laughed and Heraclitus wept.
Letter to Sir Horace Mann (31 December 1769)
Variant: The world is a comedy to those that think; a tragedy to those that feel.

As quoted in his obituary in The Guardian (28 December 1977)

Source: I Have Chosen to Stay and Fight

La vie est une tragédie pour celui qui sent, et une comédie pour celui qui pense.
As quoted in Selected Thoughts from the French: XV Century-XX Century, with English Translations (1913), pp. 132-133, by James Raymond Solly. This may conceivably be a misattribution, because as yet no definite citation of a specific work by La Bruyère has been located, and the statement is very similar to one known to have been made by Horace Walpole in a letter of 31 December 1769: The world is a comedy to those that think; a tragedy to those that feel.

Letter 123 To Robert Jephson (13 July 1777)

Source: Wise Children (1991), ch. 5 (p.213).