The right answer is that it creates a funny moment. This excerpt is part of a humorous dialogue between Jack and Algernon. Algernon wants to know why Jack's cigarette case bears the inscription "From little Cecily with her fondest love," since Jack wants to marry Algernon's cousin Gwendolen. At first Jack denies knowing anybody with that name. He then says she is his aunt, but Algernon does not believe him, since, as he reveals, the full inscription reads "From little Cecily with her fondest love to her dear Uncle Jack." If she is his aunt, why does she call him uncle? And why does she call him Jack, if Algernon has always known him as Ernest? Jack replies that he has two names, one for the city and for the countryside, but, since he does not explain why his "small aunt" calls him "dear uncle," Algernon finally says: "Come on, old boy, you had much better have the thing out at once." This comment reminds Jack of the way a dentist would address a patient, hence his witty reply.
