The Mysterious Stranger by Mark Twain
"What and ass you are!" he said. "Are you so unobservant as not to have found out that sanity and happiness are an impossible combination? No sane man can be happy, for to him life is real, and he sees what a fearful thing it is. Only the mad can be happy, and not many of those. The few that imagine themselves kings or gods are happy, the rest are no happier than the sane.Of course, no man is entirely in his right mind at any time..."