Montresor tricks Fortunado into going into the wine cellar by telling him that he wants his advice on a cask of rare wine.
Montresor’s goal is to get Fortunado into the catacombs so that he can kill him. He believes that Fortunado has insulted him extremely, and deserves to die. What the insult is, and why Fortunado deserves to be murdered for it, is never revealed.
The bait that Montresor uses to get Fortunato into the underground wine cellar in the Montresor crypt is a cask of rare Amontillado wine. He knows that Fortunato won't be able to resist.
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 connoisseur-ship in wine.
He tries to make it seem like it’s no big deal. In fact, he even goes so far as to suggest that he will get another man, Luchesi, to look at the wine. He knows this will make Fortunato more interested. He is right!
“Let us go, nevertheless. The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchesi, he cannot distinguish Sherry from Amontillado.”
Fortunato laughs of Montresor’s comments about his health. Montresor is basically using reverse psychology, trying to put Fortunato at ease. He knows that if he pretends that he does not want Fortunato to come, he will want to come even more. So he makes comments about his health, and says he will have someone else look at the wine, and Fortunato wants to come desperately because his ego is stroked.
Another reason it is so easy to get Fortunato into the cellar is because he is drunk, since “he had been drinking much.” He has been partying since it is carnival, an Italian holiday from celebrating after Lent. This just makes him easy pickings for Montresor.
Montresor uses his knowledge of human nature, and Fortunato, to get Fortunato into the cave. He realizes that if he tells him not to go, he will want to go even more. He also knows that if he pretends to worry about his health, he will never suspect he is about to be murdered. Just because you are crazy doesn't mean you aren't also smart!