Unsatisfied desire is the characteristic feature of human life. That is the common fact out of which both pessimism and optimism are constructed. Dwell on the impossibility of ever getting a state of complete and permanent satisfaction with what you have, and you become a pessimist. Dwell on the opportunity for endless growth and conquest which this same fact makes possible, and you become an optimist.
The world we live in is a world of mingled good and evil. Whether it is chiefly good or chiefly bad depends on how we take it. To look at the world in such a way as to emphasize the evil is the art of pessimism. To look at it in such a way as to bring out the good, and throw the evil into the background, is the art of optimism. The facts are the same in either case. It is simply a question of perspective and emphasis.
WILLIAM DEWITT HYDE, The Art of Optimism