Gratitude is born in hearts that take time to count up past mercies.

Charles E. Jefferson (1860 - 1937)
Gratitude is not only the greatest of virtues, but the parent of all others.

Cicero (106 BC - 43 BC), 'Pro Plancio,' 54 B.C.
Gratitude is merely the secret hope of further favors.

Francois de La Rochefoucauld (1613 - 1680)
Gratitude is the most exquisite form of courtesy.

Jacques Maritain (1882 - 1973), Reflections on America, 1958
There but for the grace of God go [I].

John Bradford, Encyclopedia of Word and Phrase Origins
It's a sign of mediocrity when you demonstrate gratitude with moderation.

Roberto Benigni (1952 - ), in Newsweek