You have asked a great question that to me highlights the criminal genius of Montresor in his deliberate plotting and leading on of Fortunato to his death. Montresor is trying to make himself sound reasonable and to not let Fortunato suspect any foul deeds are in the offing, so he deliberately tries to appear to persuade Fortunato to turn back. However, if we examine what he says closely, we know that Montresor knows the weakness of his enemy and is using it against him to lead him on to his doom.
Let us consider what Montresor tells us about Fortunato:
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 connoisseurship in wine.
It is this fatal weakness, this Achilles heel, that Montresor exploits. Note how he uses the Amontillado as bait to lure Fortunato down. What he cunningly does is to suggest that he should go to Luchesi, another noble, instead of Fortunato, as Fortunato is obviously so busy. Of course, suggesting that somebody else give information on what is Fortunato's area of expertise only strengthens Fortunato's determination to go with Montresor. Any further doubts that Montresor expresses are clear to use the same strategy:
"Come," I said, with decision, "we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi -"
Note the final four words. You see, part of Montresor's cunning is that, whilst ostensibly being concerned for his "friend's" health, he is actually showing that concern in a way which can only tempt Fortunato to go further into the catacombs with Montresor to his doom.
to say in summary, i think he does that because the author wants to show the pride and selfishness of Fortunato. and what causes his death is his pride.