I'm trying to find a quote essentially relating the sentiment that "a mind which questions the point of continuing to live, is a mind which is mentally ill", as a definition of mental illness. I thought it was Freud, but I read three pages of quotes of him and didn't find what I was looking for. I found two that are close in meaning:
No other technique for the conduct of life attaches the individual so firmly to reality as laying emphasis on work; for his work at least gives him a secure place in a portion of reality, in the human community. The possibility it offers of displacing a large amount of libidinal components, whether narcissistic, aggressive or even erotic, on to professional work and on to the human relations connected with it lends it a value by no means second to what it enjoys as something indispensible to the preservation and justification of existence in society.
SIGMUND FREUD, Civilization and Its Discontents
“About a third of my cases are suffering from no clinically definable neurosis, but from the senselessness and emptiness of their lives. This can be defined as the general neurosis of our times.” ― C.G. Jung