To answer this question, we must first consider the relationship between the narrator, Montresor, and Fortunato. Although Monstresor is careful to give Fortunato no "cause to doubt [his] good will," Montresor has a thirst for revenge due to the insults (again, unspecified) that Fortunato has allegedly hurled at him.
When Montresor runs into Fortunato at dusk one evening during the carnival season, he is thrilled to see him because he may now finally carry out the devious plan that he has been dreaming up: to lure Fortunato to his wine cellar under the precipice of tasting a fine Amontillado and to then murder him. Montresor puts on a good face so that Fortunato "did not perceive that to smile now was at the thought of his immolation."
This plan works because Montresor knows Fortunato's weakness for wine; he describes him as one of the few Italians with a "true virtuoso spirit." After luring Fortunato to his cellar and getting him drunk, Montresor paves a wall around Fortunato, effectively trapping the man so that he may suffocate to death in the damp belly of the cellar.