Doing Only and Wholly What You Must Do

What you must do is not only what you consider your duty and responsibility. It is that, but as you make choices in life, and go down certain paths, you come more and more to a place at which you recognize that what you must do, is what lies deep within you ... you must do it, because if you do not, you commit treason; you betray yourself.

As a man's real power grows and his knowledge widens, ever the way he can follow grows narrower: until at last he chooses nothing, but does only and wholly what he must do.
Ursula le Guin (The Wizard of Earthsea)