It is the formidable character of the species to routinely seek the improbable, the difficult, even the impossible, as a source of pleasure and self-justification. Who would try to write poems, or novels, or paint pictures unless he is an optimist.
Lionel Tiger

The essence of optimism is that it takes no account of the present, but it is a source of inspiration, of vitality and hope where others have resigned; it enables a man to hold his head high, to claim the future for himself and not to abandon it to his enemy.
Dietrich Bonhoeffer