The narrator, Montresor, is angry with Fortunato because he feels that Fortunato has injured and insulted him. He says, "The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge." It sounds as though Fortunato has wounded Montresor many times, but something has changed now, and it is perhaps that Fortunato has wounded Montresor's pride; this new insult was the straw that broke the camel's back.
It isn't hard to believe that Fortunato would be willing to insult Montresor, as we see him all too willing to do so during the story. He doesn't believe that the wine Montresor purchased without consulting him could actually be Amontillado, and he says, "'You have been imposed upon. And as for Luchesi, he cannot distinguish Sherry from Amontillado.'" Thus, Fortunato insults the discernment and taste of both Montresor and Luchesi, another locally-respected wine connoisseur. Then, as he and Montresor walk further into the catacombs, Fortunato seems to take pleasure in pointing out that Montresor is not a member of the brotherhood of Freemasons. When Montresor says that he is a mason, Fortunato doesn't believe him, and says, "'You? Impossible! A mason?'" He clearly wishes to lord over Montresor the fact that he belongs to this ancient brotherhood and Montresor does not, and his only motivation can be to insult Montresor again.