Clearly, the specific reasons why Montresor feels that Fortunato has injured and finally insulted him are not stated in the story. We only know that Montresor has taken great offense and has created a diabolical plan to gain revenge. The story does contain some clues as to Montresor's personality and family history that no doubt played a role in his anger and actions.
Some casual and sarcastic comments to Fortunato suggest that Montresor feels great jealousy toward him and resents his standing and success. We also learn that the coat of arms for the Montresor family reads as follows: Nemo me impune lacessit ["No one insults me with impunity."] Montresor, then, comes from a family heritage that embraces both pride and payback. They seem to have been people who were perhaps quick to take offense against perceived slights. If so, and if Montresor inherited this family nature and philosophy, then perhaps poor Fortunato was not guilty of much of anything, except in Montresor's mind.
The answer to this question can be found in the first few lines of the story itself, and more fully fleshed out with a little bit of inference, or guessing. Montresor opens the story by stating that he had planned revenge, and he stated that he was upset at Fortunado for two main reasons. He states:
"THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge."
So, here he states two reasons: 1. "Thousand injuries," and 2. "insult". So, apparently, Fortunado had inflicted thousands of injuries upon Montresor. He probably doesn't mean literal physical injuries, probably more injuries to his pride, teasing, taunting, mocking, things of that sort. So, for some reason that is not clear nor stated in the text, Montresor felt that Fortunado had slighted and injured him over and over again. We don't know exactly what he did, but can guess that Fortunado probably just made Montresor feel offended or slighted--whether intentionally or simply from being clueless. The second reason, insult, was probably a more direct insult directed at Montresor, but, they two seem like pretty good pals when the meet to discuss the wine, so Fortunado can't hold too much of a grudge against him. But, Montresor feels insulted nevertheless, and vows revenge.
The revenge he enacts is cruel and vicious indeed, and one has to wonder whether or not the bumbling Fortunado actually deserved it. I hope that those thoughts help a bit; good luck!