“I think to feel this happy is to be miserable, to feel this much satisfaction is to burn.”

Source: Mansfield Park

Tom Masson in: The Century Illustrated Monthly Magazine, Vol. 61 (1901). p. 319.

Source: The Happiness Project: Or Why I Spent a Year Trying to Sing in the Morning, Clean My Closets, Fight Right, Read Aristotle, and Generally Have More Fun

Wording in Ideas and Opinions: Everything that the human race has done and thought is concerned with the satisfaction of deeply felt needs and the assuagement of pain. One has to keep this constantly in mind if one wishes to understand spiritual movements and their development. Feeling and longing are the motive force behind all human endeavor and human creation, in however exalted a guise the latter may present themselves to us.
1930s, Religion and Science (1930)
Context: Everything that men do or think concerns the satisfaction of the needs they feel or the escape from pain. This must be kept in mind when we seek to understand spiritual or intellectual movements and the way in which they develop. For feelings and longings are the motive forces of all human striving and productivity—however nobly these latter may display themselves to us.

Daily mantra for positive self-suggestion which he recommended in Success Through a Positive Mental Attitude (1960) and his teachings.

As quoted in "BiH: Predstavljen prijedlog teksta državne himne" https://ba.voanews.com/a/a-29-2009-02-20-voa3-86124032/679893.html (20 February 2009), VOA News
2000s

Gli uomini quando si sentono felici pensano agli uomini che amano, quando si sentono tristi pensano e pregano Dio.

Quote in his letter to brother Theo from Auvers, July 1890; as quoted in Vincent van Gogh, edited by Alfred H. Barr; Museum of Modern Art, New York, 1935 https://www.moma.org/documents/moma_catalogue_1996_300061887.pdf, (letter 648), p. 26
1890s