“Happiness comes from the full use of one's power to achieve excellence.”

Thoughts on Man's Purpose in Life (1974)
Context: Happiness comes from the full use of one's power to achieve excellence. Life is potentially an empty hole, and there are few more satisfying ways of filling it than achieving and exercising excellence.
This principle of excellence is on which Americans seem to be losing, and at a time when the Nation stands in need of it. A lack of excellence implies mediocrity. And in a society that is willing to accept a standard of mediocrity, the opportunities for personal failure are boundless. Mediocrity can destroy us as surely as perils far more famous.
It is important that we distinguish between what it means to fail at a task and what it means to be mediocre. There is all the difference in the world between the life lived with dignity and style which ends in failure, and one which achieves power and glory, yet is dull, unoriginal, unreflective, and mediocre. In a real sense, what matters is not so much whether we make a lot of money or hold a prestigious job; what matter is that we seek out others with knowledge and enthusiasm — that we become people who can enjoy our own company.