All previous experience should teach him that it is not safe to be too happy, that he cannot live on the heights of joy for too long with impunity. It is not safe to exult too freely in the good fortune which comes in the summers of life; it is not safe to forget the hours of bad fortune which came in the winters of life. Fate cannot be trusted to bring in only such pleasant hours, for it may equalize itself by hurting him now and then. He should temper his delight at fate with fear of it. But even this is not an ideal attitude. Serenity, which leaves him above both delight and fear, is immensely better.
-- Notebooks Category 24: The Peace within You > Chapter 1: The Search for Happiness > # 62