Why does Montresor want revenge against Fortunato?

Montresor begins his narrative by stating

The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.

Since Montresor never describes the insult or any of the injuries, readers have offered all sorts of conflicting opinions about his motivation. Some believe that Montresor was never injured or insulted at all and therefore must be insane. Poe avoids having to clarify or justify Montresor's motivation by having him address his communication, or confession, or letter, to a person he calls 

You, who so well know the nature of my soul

Presumably this person knows so much about Montresor that it is not necessary for him to give any examples of the injuries. This device is effective because it forces the reader to pay close attention to the text in an effort to deduce facts that are fully understood by "You, who so well know the nature of my soul." This is very much like Ernest Hemingway's famous "Iceberg principle." 

If a writer of prose knows enough of what he is writing about he may omit things that he knows and the reader, if the writer is writing truly enough, will have a feeling of those things as strongly as though the writer had stated them. The dignity of movement of an ice-berg is due to only one-eighth of it being above water. A writer who omits things because he does not know them only makes hollow places in his writing.
                         Ernest Hemingway, Death in the Afternoon

It does not seem likely that Montresor could be insane and write such a coherent narrative. He must be sane and telling the exact truth. There should be indications within the narrative of the types of injuries that have driven Montresor to plan and execute his revenge. The injuries would have to be of a kind that are known only to Montresor and Fortunato. Montresor wants to kill with "impunity," and he could not expect impunity if it were widely known that he had been injured by Fortunato anything like a thousand times. Poe offers a clue in the third paragraph of the story, which should be read with special attention.

He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseur-ship in wine. 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 painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially;—I was skilful in the Italian vintages myself, and bought largely whenever I could.

This sounds as if both men are "gentlemen-brokers" who earn their livings by dealing in luxury goods such as paintings, jewelry,antiques, gourmet wines--even in real estate. Fortunato is rich, Montresor is poor. Montresor puts up with Fortunato's injuries because he is dependent on him financially. There are many impoverished Venetian aristocrats who are forced to dispose of family heirlooms in order to survive in their decaying palazzi. Montresor may often need to borrow from the man he constantly describes as his good friend in order to purchase an item for resale. Or he may go into an ad hoc partnership with Fortunato on a purchase. Or he may simply receive a finder's fee for introducing his good friend to a prospective buyer or seller of some one-of-a-kind family treasure. Fortunato would have plenty of opportunities to "injure" Montresor, without anyone else knowing about it, by taking an unfair share of the profits on a transaction, by paying a lower finder's fee than agreed upon, or cheating him in a dozen other ways. As Montresor says to his good friend when they are in the catacombs:

“Come,” I said, with decision, “we will go back; your health is precious.You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter."

Montresor knows that Fortunato is planning to cheat him on the nonexistent Amontillado. It would be just the sort of opportunity Fortunato could not resist. He is planning to taste the wine and, assuming it is genuine, tell Montresor it is only ordinary sherry, then find the nonexistent Spanish ship with its nonexistent cargo of Amontillado and buy up the whole shipload. When Montresor found out what happened, Fortunato would laugh it off as "an excellent jest." He is a scoundrel, but he considers himself a funny fellow, which is why he wears a jester's costume in the carnival.

This is a logical explanation of why Montresor, who is extremely clever and perfectly sane, should want revenge against Fortunato. Montresor has been cheated by this man so many times that he knows he can entrap him simply by offering him an apparent opportunity to cheat him once again.