The narrator of "Cask of Amontillado, wants Fortunato to go to the narrator's vaults - so why say the opposite to Fortunato?
The narrator, Montresor, uses a bit of reverse psychology on Fortunato. He understands his prey very well, and has long planned for this ambush. He starts by mentioning something that Fortunato is very interested in - the Amontillado. Then he plays upon Fortunato's natural jealousy and possessiveness by saying that he, Montresor, will get Luchesi to verify the Amontillado. After this mention, Fortunato is anxious to have access to the wine. It becomes an urgent matter, and Fortunato presses Montresor to show him the "pipe." By remaining somewhat resistant, and by expressing concern for Fortunato's health (the basement is damp), Montresor both increases Fortunato's urgency and makes himself seem harmless. Here is the last exchange from the story, when Montresor exclaims his concern for Fortunato:
“My friend, no. It is not the engagement, but the severe cold with which I perceive you are afflicted. The vaults are insufferably damp. They are encrusted with nitre.”
“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.”