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) Irish 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."