This is a question that has puzzled readers of Edgar Allan Poe's short story, "The Cask of Amontillado," ever since its publication. It is likely that Poe deliberately did not give more information or a specific reason about the relationship between the two characters, probably to maintain the mystery that surrounds the two men. We know that the two, Fortunato and Montresor, are acquaintances, and that they come from old, wealthy families. As for Montresor's hatred, we are only given two hints: We know, from the first line of the story, that
THE THOUSAND INJURIES of Fortunato I had borne as I best could...
The injuries are never explained by Montresor, and there is no further mention of them in the story. We also know that
... when he ventured upon insult I vowed revenge.
Again, the specific insult is never identified, but judging from Montresor's decision to kill Fortunato for the offense, it must have been fairly serious.