Why does Montresor want revenge in The Cask of Amontillado?

According to Montresor's opening comments he has suffered a thousand injuries at the hands of Fortunato but now Fortunato has also insulted him. However, it is not clear if these injuries and insults are real or imagined. He gives no details and later Fortunato greets him in a very friendly way, offers to leave the carnival to test the wine for Montressor and insists on continuing through the passageway in the cellar. Although Fortunato is partly driven by his own pride and vanity, these are hardly the actions of a person who has committed a thousand injuries and insults against Montressor.