This is one of the aspects that Edgar Allan Poe never reveals in his classic gothic short story, "The Cask of Amontillado." We know that the narrator, Montresor, has been slighted in some manner by his nemesis, Fortunato, but Montresor never specifically reveals what the "thousand injuries" are. Additionally, we also know that Fortunato has added insult ("he ventured upon insult") to injury, thus causing Montresor to plot his special type of revenge. There is absolutely no other hint concerning Fortunato's sins against Montresor, but we can assume that the narrator is exaggerating with the number of injuries in question.
Poe was a perfectionist. It is hard to believe that he would provide Montresor with such a strong motivation for refenge without giving some clue to the "thousand injuries" he had suffered. Poe did not need to describe any of these injuries in the opening of his story, which would have involved more exposition. He knew he could demonstrate Fortunato's faults anywhere in the story. In fact, the injurious behavior is so blatant that it can easily be overlooked by the reader.
Fortunato is rich. Montresor is poor. Both men apparently deal in luxury goods with millionaires. Here are a couple of significant sentences in the third paragraph of the story:
Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity to practise imposture upon the British and Austrian millionaires.
In that same paragraph Montresor says:
I was skilfull in the Italian vintages myself, and bought largely whenever I could.
This does not mean that he bought largely whenever he could find good wines for sale but that he bought largely whenever he could afford to. Both men bought art works, jewels, antiques, and other luxury goods including fine wines to sell at a profit to rich foreigners. Fortunato was rich and could afford to outbid Montresor continually. It was in business deals that Montresor suffered most of his injuries.
Note that Montresor does not consider himself an Italian although he lives in Italy. He has a French name. He is not taking part in the Italian carnival. He is wearing a French style cloak. He gives his victim French wine twice when they are underground. His family may have lived in Italy for some time, but he could still be considered an outsider by the aristocracy, and Fortunato might have taken cruel pleasure in reminding him of his outsider status. There are a great many human bones in the catacombs. This could be taken to show that the Montresors have lived there for many centuries--but it is never stated that the bones belong to Montresor's ancestors.
Fortunato is not interested in the Amontillado because he wants to drink some of it or to show off his connoisseurship. He senses that he might make a big profit. Montresor only bought one cask (a "pipe" containing 126 gallons), but Fortunato could afford to buy a whole shipload. First he must establish that it is the true Amontillado. He doesn't want Montresor to go to Luchesi because Luchesi is a competitor. For that matter, Montresor himself might become a competitor. He twice tells Fortunato that he has his doubts about the authenticity of the Amontillado. If Fortunato verifies that it is genuine, then Montresor might go back to his source and buy some more. The suggestion in his saying, "I have my doubts" is that he only bought one cask because he wasn't sure it was the real Amontillado. And Montresor is well aware that Fortunato is thinking of beating him out of a good deal once again if the wine is authentic. Montresor understands Fortunato's ruthless character and hates him for it while using it against him.