I've quoted below from writer Benjamin Moser's thoughtful "Bookends" essay for The New York Times, January 27, 2015.
"We never know if we are doing it right. Even the best writing will never have the immediate, measurable impact that a doctor's work has, or a plumber's. To discover if we are on the right track, we can, and do, become obsessed with our "careers," which is the word we use for what other people think of us...there is something dreary about wanting writing to be a real job. The sense of inner purpose, so often unmentionable in a society enamored of professionalization, distinguishes a writer from a hack...a writer--independent of publication or readership or "career"--is always a writer. Independent, even of writing. Writing, after all is something on does. A writer is something one is."