Proust no doubt approved of Tolstoy’s opening to Anna Karenina: “Happy families are all alike; every unhappy family is unhappy in its own way.” The only difference is that for him unhappiness is not so much a one way trip to the train station, but the fruit of happiness.
As for happiness, that is really useful to us in one way only, by making unhappiness possible. It is necessary for us to form in happiness ties of confidence and attachment that are both sweet and strong in order that their rupture may cause us the heart-rending but so valuable agony which is called unhappiness. Had we not been happy, if only in hope, the unhappinesses that befall us would be without cruelty and therefore without fruit. (VI,316)
There is cruelty because there is love. This drives the author to new depths of feeling.
…it almost seems as though a writer’s works, like the water in an artesian well, mount to a height which is in proportion to the depth to which suffering has penetrated his heart. (VI,318)
And once one understands that suffering is the best thing that one can hope to encounter in life, one thinks without terror, and almost as of a deliverance, of death. (VI,319)