Man has gradually become a fanciful animal, who has one more condition of existence to fulfil than any other animals: from time to time, man must think he knows why he exists; the human race cannot flourish without periodically renewed trust in life! Without believing in the reason in life!

[Der Mensch ist allmählich zu einem phantastischen Thiere geworden, welches eine Existenz -Bedingung mehr, als jedes andere Thier, zu erfüllen hat: der Mensch muss von Zeit zu Zeit glauben, zu wissen, warum er existirt, seine Gattung kann nicht gedeihen ohne ein periodisches Zutrauen zu dem Leben! Ohne Glauben an die Vernunft im Leben!]

Friedrich Nietzsche (1844-1900) German philosopher and poet

The Gay Science [Die fröhliche Wissenschaft], Book 1, § 1 (1882) [tr. Hill (2018)]
    (Source)

Also known as La Gaya Scienza, The Joyful Wisdom, or The Joyous Science.

(Source (German)). Alternate translations:

Man has gradually become a visionary animal, who has to fulfil one more condition of existence than the other animals: man must from time to time believe that he knows why he exists; his species cannot flourish without periodically confiding in life! Without the belief in reason in life!
[tr. Common (1911)]

Gradually, man has become a fantastic animal that has to fulfil one more condition of existence than any other animal: man has to believe, to know, from time to time why he exists; his race cannot flourish without a periodic trust in life -- without faith in reason in life.
[tr. Kaufmann (1974)]

Man has gradually become a fantastic animal that must fulfil one condition of existence more than any other animal: man must from time tot time believes he knows why he exists; his race cannot thrive without a periodic trust in life -- without faith in the reason in life!
[tr. Nauckhoff (2001)]