Peter Drucker, from his article “Managing Oneself” (pdf):
Successful careers are not planned.
They develop when people are prepared for opportunities because they know their strengths, their method of work, and their values. Knowing where one belongs can transform an ordinary person — hardworking and competent but otherwise mediocre — into an outstanding performer.
A helpful resource that fleshes this out is Daniel Pink’s The Adventures of Johnny Bunko: The Last Career Guide You’ll Ever Need.