Why does Montresor hate Fortunato in "The Cask of Amontillado"?
At the beginning of the short story, Montresor mentions that Fortunato has caused him a "thousand injuries" and insulted his family's prestigious name. Interestingly, Montresor does not elaborate on how Fortunato harmed him a thousand times, but the reader can surmise that Fortunato verbally insulted Montresor publicly or offended his family's name after analyzing Fortunato's character. Montresor describes Fortunato as being respected and feared by his neighbors, friends, and associates. Montresor also characterizes Fortunato as a confident, arrogant man. Once Montresor persuades Fortunato to follow him into his catacombs, Fortunato mentions that the vaults are extensive. Montresor quickly reminds him that his family is "great" and "numerous" before elaborating on his family's coat of arms. Montresor's response reveals his pride in his family's name, which is further evidence that he seeks revenge because Fortunato has offended his family. Overall, Montresor does not directly address why he hates Fortunato, but one can surmise that his hate stems from Fortunato publicly insulting his family's name or verbally abusing him.
The short answer is, we don't really know.
The somewhat longer answer is, we only get sweeping generalities at the start of the story, and then a few times later. We know he says Fortunato had done him a "thousand injuries," but we don't learn what they are. We know he sees Fortunato as having insulted him--but we don't know what the insult is, and Fortunato seems to think they are on good terms. Therefore, we'll have to say that Montressor's insane pride was hurt, and that's all we know.