from:" cask of Amontillado"
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Montressor needs to make sure that his plan goes off without a hitch. If Montressor were to act indifferent to Fortunato's health, Fortunato may suspect that Montressor was up to no good. Montressor is simply trying to put Fortunato at ease, and play the concerned friend who would never do anything to harm his "friend" Fortunato--see, Montressor is even concerned for Fortunato's health. There's no reason Fortunato shouldn't follow this man deeper and deeper into the underground.
It also serves as a delicious irony for Montressor. He can act concerned about Fortunato's health, even though Montressor knows that Fortunato's cough and general health are the least of his concerns right now and will not be the cause of Fortunato's death.