Only one endowed with restless vitality is susceptible to pessimism. You become a pessimist --a demonic, elemental, bestial pessimist --only when life has been defeated many times in its fight against depression.
Pessimism is, in brief, playing the sure game. You cannot lose at it; you may gain. It is the only view of life in which you can never be disappointed. Having reckoned what to do in the worst possible circumstances, when better arise, as they may, life becomes child's play.
It is a quite remarkable fact that the great religions of the most civilized peoples are more deeply fraught with sadness than the simpler beliefs of earlier societies. This certainly does not mean that the current of pessimism is eventually to submerge the other, but it proves that it does not lose ground and that it does not seem destined to disappear.