From his introduction to the 1949 edition of Goethe’s autobiography, Truth and Fantasy from My Life:
It seems that two qualities are necessary if a great artist is to remain creative to the end of a long life; he must on the one hand retain an abnormally keen awareness of life, he must never grow complacent, never be content with life, must always demand the impossible and when he cannot have it, must despair. The burden of the mystery must be with him day and night.
[The artist] must be shaken by the naked truths that will not be comforted. This divine discontent, this disequilibrium, this state of inner tension is the source of artistic energy. Many lesser poets have it only in their youth; some even of the greatest lose it in middle life. Wordsworth lost the courage to despair and with it his poetic power. But more often the dynamic tensions are so powerful that they destroy the man before he reaches maturity.
By way of Brain Pickings.