“Men can only be happy when they do not assume that the object of life is happiness.”

To the Republican Citizens of Washington County, Maryland (31 March 1809)
1800s, Post-Presidency (1809)

Independence Day speech (1828)
Context: Americans no longer argue on the propriety of making all men soldiers, in order that their nation may be an object of terror to the rest of the world. They understand that the happiness of a people is the only rational object of a government, and the only object for which a people, free to choose, can have a government at all. They have, farther, almost excluded war as a profession, and reduced it from a system of robbery to one of simple defence. In so doing, they ought also to have laid aside all show of military parade, and all ideas of military glory. If they have not done so, it is that their reform in this matter is yet imperfect, and their ideas respecting it are confused.

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:

Fifteen hours with Fabio http://www.washingtonpost.com/sf/style/2015/12/23/fabio/?utm_term=.55d4ac289b9c (December 23, 2015)

What Is To Be Done? (1886) Chap. XXXVIII, as translated in The Novels and Other Works of Lyof N. Tolstoï (1902) edited by Nathan Haskell Dole, p. 259

Source: The enjoyments of life (such was now my theory) are sufficient to make it a pleasant thing, when they are taken en passant, without being made a principal object. Once make them so, and they are immediately felt to be insufficient. They will not bear a scrutinizing examination. Ask yourself whether you are happy, and you cease to be so. The only chance is to treat, not happiness, but some end external to it, as the purpose of life. Let your self-consciousness, your scrutiny, your self-interrogation, exhaust themselves on that; and if otherwise fortunately circumstanced you will inhale happiness with the air you breathe, without dwelling on it or thinking about it, without either forestalling it in imagination, or putting it to flight by fatal questioning. This theory now became the basis of my philosophy of life. And I still hold to it as the best theory for all those who have but a moderate degree of sensibility and of capacity for enjoyment; that is, for the great majority of mankind."

Autobiography, Ch 5, https://www.gutenberg.org/files/10378/10378-h/10378-h.htm#link2H_NOTE https://www.laits.utexas.edu/poltheory/mill/auto/auto.c05.html source: Autobiography (1873), Ch. 5: A Crisis in My Mental History (p. 100)

Mount Madonna Messenger, March 26, 2016

As quoted by Ernst Straus in Einstein: A Centenary Volume by A.P. French (1980), p. 32.
Attributed in posthumous publications
Variant: "if you want to be a happy man, you should tie your life to a goal, not to other people and not to things." A quote from Ernst Straus' memoir of Einstein in Albert Einstein: Historical and Cultural Perspectives edited by Gerald Holton and Yehuda Elkana (1982), p. 420 http://books.google.com/books?id=CNuwE3NL1QgC&lpg=PP1&pg=PA420#v=onepage&q&f=false