Explain this quote: Every person who has mastered a profession is a skeptic concerning it.

I think all these answers are good and touch on possibilities. Shaw's assertion overreaches, however, when he says "every." Having mastered several usual and unusual professions myself, now that I'm old I don't think it applies to every profession. His assertion is also rather peculiar in that "skeptic" is a slippery and often misunderstood word: it does not mean "cynic," one who discounts new ideas or thinks the future must turn out badly, as is often assumed.

As Shaw was British, I consulted the OED to give a better idea of the sense of the word in his mind. Concise Version OED 2008 states: a person inclined to question or doubt accepted opinions. Ah hah!

In one's own mastered profession(s) I find that one can question or doubt:

-it's real value to others and was this value been overrated?

-it being as satisfying at the end of the years, or as glamorous as one imagined, when younger and starting out. It is probably disappointing in several respects, as many things turn out to be with time. As has been said, in the beginners mind there are many possibilities; in the expert's, few. And most of us habituate to many repeated exciting stimuli and experiences, they become humdrum. Or years of sex with the same person. A tour guide at Niagara Falls or other amazing attraction will not experience the excitement or amazingness after years of seeing it again and again. This is how most of us are wired.

-Now it is up to you to also think of counterexamples. We've all heard the old saw "Nobody on his deathbed says 'I wish I had spent more time at the office.'" Well, how about a medical researcher such as Dr. Jonas Salk, who came up with the cure for polio? Do you think he was questioning or doubting his or his work's value?

I have mastered two specialty professions and skills which I now volunteer at my local animal shelter. Am I skeptical in any sense whatsoever about them? No.

As with many quote originators, including myself, my guess is that Shaw came up with this one by discovering it in himself.