Why does the narrator, Montresor, never reveal the specific insult that prompted him to seek revenge against Fortunato in "The Cask of Amontillado"?

This is a question which every single reader of "The Cask
of Amontillado" must ask himself. Obviously, Edgar Allan Poe never gives us a specific
reason, only that there had been "the thousand injuries of Fortunato," and that, at some
point, Montresor's adversary had gone too far, forcing the narrator to take revenge.
Perhaps Montresor (or Poe) chooses not to reveal the exact insults to maintain a
mysterious aura; it could also be because Montresor is retelling the story 50 years in
the future, and the reasons are either unimportant or long forgotten. Since Montresor's
narration almost takes on a sense of confessional (perhaps to a priest or relative), his
goal may be to only confess his own crime, and not Fortunato's.