Why does Montresor decide to seek revenge against Fortunato in "The Cask of Amontillado"?

Montresor,
Poe's unreliable and hyperbolic narrator, claims that he seeks revenge after Fortunato has added
insult to injury.

In theof Poe's Gothic tale, Montresor claims that he has
endured "the thousand injuries" that Fortunato has committed against him; however,
when his enemy has "ventured upon insult," he states that he can bear no more, and
must be avenged. Having decided upon revenge, Montresor commences his intricate plan to approach
Fortunato during the Carnival season when Fortunato's disappearance should not soon be noticed.
Also, Fortunato, who should be at least somewhat inebriated from celebrating, will be more
susceptible to Montresor's luring him into the catacombs on the pretext of tasting the
Amontillado. 

Montresor's plan is effective as he succeeds in tempting his
enemy Fortunato into the damp "vaults." Further, Montresor exploits Fortunato's desire
to outdo his rival Luchesi by tasting the Amontillado. Montresor also feigns concern for
Fortunato's health because of the dampness of the cavern walls and repeatedly suggests that they
turn back. But Fortunato, who will not be outdone later by Luchesi or anyone else, insists that
they keep going forward. As Montresor knows, Fortunato is a rapacious man who wishes to taste
the Amontillado and judge it before his foe Luchesi has any chance to do so.