“Gratitude is a euphemism for resentment.”
Source: Stranger in a Strange Land
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Robert A. Heinlein 557
American science fiction author 1907–1988Related quotes
“The greatest antidepressant is gratitude.”
Source: Life, the Truth, and Being Free (2010), p. 124

“Gratitude is a sickness suffered by dogs.”
As quoted in The Memoirs of Stalin's former secretary (1992) by Boris Bazhanov [Saint Petersburg] (in Russian) http://lib.ru/MEMUARY/BAZHANOW/stalin.txt
Contemporary witnesses

“Revenge is profitable, gratitude is expensive.”
Vol. 1, Chap. 11.
The Decline And Fall Of The Roman Empire: Volume 1 (1776)
Source: The Decline and Fall of the Roman Empire

“Joy is the simplest form of gratitude.”
As quoted in Finding the Magnificent in Lower Mundane : Extraordinary Stories About An Ordinary Place (1994) by Bob Stromberg, p. 69.

“Gratitude is riches. Complaint is poverty.”
Though she is quoted as saying this in a 1996 interview, she is quoted as saying it is a maxim which she follows as a Christian Scientist, and it seems to come from words of a Christian Science Hymn. It does come from Hymn 249 in the Christian Science Hymnal
Misattributed