In "The Cask of Amontillado," for what reason does Montresor say he does not want to impose upon Fortunato's good nature?

Montresor is trying to trick Fortunato. He wants to lure him into the cellar, on the face of it to taste a cask of the finest amontillado. But of course his plans are more sinister; he wants to kill Fortunato for various (unspecified) wrongs. To make his ruse more effective, it is necessary for Montresor to give the impression that he likes Fortunato and wants to be friends with him. He also wants to make it seem as if he really doesn't want Fortunato to try the wine—that he's trying to dissuade him from venturing down into the cellar:

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.

Montresor is being very sneaky here. He knows that Fortunato will be more curious about the wine if he senses that Montresor is trying to discourage him from visiting the cellar. He also plays on Fortunato's vanity as a wine connoisseur. Montresor strategically says he wouldn't want to put Fortunato to any trouble; plus, it will be so horribly cold and damp down there. He says maybe he should ask Luchresi for his opinion instead, to which Fortunato replies:

Let us go, nevertheless. The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchresi, he cannot distinguish Sherry from Amontillado.

The ruse works. And tragically for Fortunato, he comes to regret being tricked by Montresor.