An easy way to answer this question would be to focus on Fortunato's obvious character trait as a heavy drinker. He is a connoisseur of wine, as Montresor states, but one does not become a connoisseur of wine without drinking a lot of wine over a long period of time. There is plenty of evidence in the story. When Montresor first encounters him on the street:
He accosted me with excessive warmth, for he had been drinking much.
Fortunato is already heavily drunk when they reach Montresor's wine vault under his palazzo.
He turned towards me, and looked into my eves with two filmy orbs that distilled the rheum of intoxication.
Montresor gives him a bottle of French wine which he consumes quickly. Then a short while later Montresor offers him another bottle.
I broke and reached him a flagon of De Grave. He emptied it at a breath. His eyes flashed with a fierce light. He laughed and threw the bottle upwards with a gesticulation I did not understand.
Fortunato must be a really heavy drinker to be able to consume two big bottles of wine "at a breath." The fact that Fortunato is such a heavy drinker works to Montresor's advantage. It is essential to the story's plot. It makes it easier for Montresor to fool his victim and lead him to the niche where he intends to chain him to the granite wall and leave him to die.