“It is the duty of man to raise up man. One is guilty of all abjection that one does not help to relieve. Only those who spread treachery, fire, and death out of hatred for the prosperity of others are undeserving of pity.”

Saknússemm II
Kristnihald undir Jökli (Under the Glacier/Christianity at Glacier) (1968)

Vol. 2, Ch. 24 "Oracular Philosophy and the Revolt against Reason"
The Open Society and Its Enemies (1945)
Context: ... the attempt to make heaven on earth invariably produces hell. It leads to intolerance. It leads to religious wars, and to the saving of souls through the inquisition. And it is, I believe, based on a complete misunderstanding of our moral duties. It is our duty to help those who need help; but it cannot be our duty to make others happy, since this does not depend on us, and since it would only too often mean intruding on the privacy of those towards whom we have such amiable intentions.

The Drowned and the Saved (1986)

Call no man happy till he is dead.
Also attributed to Sophocles in "Oedipus The King".
Hold him alone truly fortunate who has ended his life in happy well-being.
Source: Oresteia (458 BC), Agamemnon, lines 928–929. Variant translations:

Source: One-Dimensional Man (1964), p. 61

'Men for Others' http://www.creighton.edu/CollaborativeMinistry/men-for-others.html, 1973, Valencia, Spain

(Often shortened to "can't stand prosperity" as an unknown quote).
1840s, Heroes and Hero-Worship (1840), The Hero as Man of Letters

Vol. 4, pt. 2, translated by W.P.Dickson.
The History of Rome - Volume 4: Part 2