“Man needs his difficulties because they are necessary to enjoy success.”

Variant: The man of virtue makes the difficulty to be overcome his first business, and success only a subsequent consideration: this may be called perfect virtue.
Source: The Analects, Other chapters

World Wildlife Fund: British National Appeal Banquet, London (1962)
The Environmental Revolution: Speeches on Conservation, 1962–77 (1978)
Context: For conservation to be successful it is necessary to take into consideration that it is a characteristic of man that he can only be relied upon to do anything consistently which is in his own interest. He may have occasional fits of conscience and moral rectitude but otherwise his actions are governed by self-interest. It follows then that whatever the moral reasons for conservation it will only be achieved by the inducement of profit or pleasure.

"The Tragedy of the Moon," The Magazine of Fantasy and Science Fiction (July 1972)
General sources

Quoted by C. S. Hastings in "Biographical Memoir of Josiah Willard Gibbs 1839-1903," National Academy of Sciences Biographical Memoirs, vol. VI (Washington, D.C.: National Academy of Sciences, 1909), p. 390. Complete memoir http://books.nap.edu/html/biomems/jgibbs.pdf
Attributed

Individualism and Socialism (1933)

Source: Meditations in Wall Street (1940), p. 104

To create for the ages, let's combine art and engineering, Bran, Ferren, January 23, 2018, www.ted.com, March 2014 https://www.ted.com/talks/bran_ferren_to_create_for_the_ages_let_s_combine_art_and_engineering,

Source: The Winner Stands Alone