 According to the text of the tale, the reason for the narrator's monomaniacal obsession with the man stems from "the absolute idiosyncrasy of [the man's] expression".He is the only person walking down the street the narrator can't categorize.Why the narrator is so haunted by him is not entirely clear, though it is implied that the two men are two sides of the same person, with the old man representing a secret side of the narrator, though the narrator is unable to see this.