why does the narrator consider his passing for white a "capital joke"?

Knowing the plight of the Negro man in America made passing for a white man, and reaping the benefits, the narrator's joke on the country.

Do I not disprove the theory that one drop of Negro blood renders a man unfit?" Many a night when I returned to my room after an enjoyable evening, I laughed heartily over what struck me as the capital joke I was playing.