“Those only are happy (I thought) who have their minds fixed on some object other than their own happiness.”

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)