Isn't duty only a second-best to keep one going until one learns to like the thing, and then it is a duty no more? When love fulfills the Law, Law (as such) flies out of the window. Isn't that part of what St. Paul meant by being free from the Law? And of what St. Augustine meant by 'Have charity and do what you like'?
--C. S. Lewis, letter to Mary Van Deusen, Dec 16, 1955 (Collected Letters of C. S. Lewis, 3:685)