You have asked a really interesting question for a number of reasons. Firstly, remember the point of view of this excellent short story. We see everything from the perspective of Montresor, whom astute readers will realise may not be the most reliable of narrators. This therefore might cause us to doubt some of what Montresor tells us about Fortunato, especially the first paragraph, when Montresor protests how much Fortunato has wronged him:
The thousand injuries of Fortunato I had borne as best I could; but when he ventured upon insult, I vowed revenge.
It is hardly likely that Fortunato, if he had insulted Montresor so badly, could be so naive about trusting himself into Montresor's hands and delving deep into the Montresor catacombs with only his enemy for a companion.
However, apart from these debatable facts, we do know that Fortunato is a wine connoisseur:
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 connoisseurship in wine.
This of course, as the narrator observes, is his Achilles heel, and is used ruthlessly by Montresor to tempt his victim down into the catacombs and to meet his revenge. The cask of Amontillado of the title is what Montresor pretends he has brought and wants Fortunato to sample for him to test its worth. Note how Montresor tempts Fortunato to sample the wine for him by saying that he is going to another Italian noble to test it for him, Luchesi:
"As you are engaged, I am on my way to Luchesi. If anyone has a critical turn it is he. He will tell me -"
This of course plays with the pride of Fortunato who considers himself an expert in wines such as Amontillado, and thus the trap is sprung and Fortunato is led captive to his fate by his one weakness - his knowledge of wine.
Note that Fortunato apart from this weakness is described as a "man to be respected and even feared". This description makes Montresor all the more remarkable for his ability to detect his enemy's weakness and plot how he can use it to bring about his downfall.