This question is a good one; it takes Holmes out of his head, and he follows along with that easily, pulling on his cigarette as he considers the question. "You ought to write a more satisfying story," he decides on easily enough, pacing over to the window and peering outside as he thinks.

He'd already written about soldiers and thieves, and another suitably adventurous, robust sort of a theme is needed for this second story -- otherwise it would not be Watson. Fortunately it isn't a hard leap from 'soldiers and thieves' to pirates, which is where Holmes's mind leads him.

"Pirates," he says, turning away from the window. "You should write about pirates."