It is no use my liking or disliking; I do what must be done, and have no time to attend myself. That is not happiness, but it is greatness.

George Bernard Shaw (1856-1950) British playwright and critic

Caesar and Cleopatra, Act IV [Cleopatra] (1898)

Full text. An variation on this is frequently quoted, but I haven't been able to find a source: "Forget about likes and dislikes. They are of no consequence. Just do what must be done. This may not be happiness, but it is greatness."