Undoubtedly we should desire the happiness of those whom we love, but not as an alternative to our own. In fact the whole antithesis between self and the rest of the world, which is implied in the doctrine of self-denial, disappears as soon as we have any genuine interest in persons or things outside ourselves. Through such interests a man comes to feel himself part of the stream of life, not a hard separate entity like a billiard-ball, which can have no relation with other such entities except that of collision.
Source: The Conquest of Happiness, 1930, chap. 17：The happy man