“The isness of things is well worth studying; but it is their whyness that makes life worth living.”

Referencing Oscar Wilde from the preface of "The Picture of Dorian Gray"; "All art is quite useless".
1990s, Moab is My Washpot (autobiography, 1997)
Source: Moab Is My Washpot
Context: … but love, like all art, as Oscar said, it's quite useless. It is the useless things that make life worth living and that make life dangerous too: wine, love, art, beauty. Without them life is safe but not worth bothering with.

Lord Illingworth, Act III
A Woman of No Importance (1893)

1900s, Address at the Prize Day Exercises at Groton School (1904)
Context: Of course, the worst of all lives is the vicious life; the life of a man who becomes a positive addition to the forces of evil in a community. Next to that and when I am speaking to people who, by birth and training and standing, ought to amount to a great deal, I have a right to say only second to it in criminality comes the life of mere vapid ease, the ignoble life of a man who desires nothing from his years but that they shall be led with the least effort, the least trouble, the greatest amount of physical enjoyment or intellectual enjoyment of a mere dilettante type. The life that is worth living, and the only life that is worth living, is the life of effort, the life of effort to attain what is worth striving for.

Variant: It is the lives we encounter that make life worth living.

Source: Even Cowgirls Get the Blues

Source: Attributed in posthumous publications, Einstein and the Poet (1983), p. 91

Women Talking Dirty

Source: Of Human Bondage (1915), Ch. 66

VIII, line 83.
Satires, Satire VIII