Montresor's attitude towards Fortunato throughout "The Cask of Amontillado" can best be described as obsequious. This word is defined in various ways. Dictionary.com defines it as:
characterized by or showing servile complaisance or deference...fawning
"Fawning" is defined as:
displaying exaggerated flattery or affection
Montresor certainly displays exaggerated affection. Although he hates Fortunato and intends to murder him in a horrible way, he calls him "My friend" and refers to him as "my good friend" and "my poor friend" throughout the tale. Here are some examples of obsequious speech and behavior intended to reassure his intended victim of his good will:
I said to him—“My dear Fortunato, you are luckily met. How remarkably well you are looking to-day."
“My friend, no; I will not impose upon your good nature."
I took from their sconces two flambeaux, and giving one to Fortunato, bowed him through several suites of rooms to the archway that led into the vaults.
“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—”
Montresor's behavior is that of an inferior towards a superior. This suggests an answer to the questions: Why did Montresor put up with a "thousand injuries"? Why didn't he just stay away from him? Montresor is dependent upon Fortunato for money. The third paragraph suggests that these two men may invest or trade in luxury items.
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 connoisseur-ship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity, to practise imposture upon the British and Austrian millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially;—I was skilful in the Italian vintages myself, and bought largely whenever I could.
Perhaps Montresor put up with Fortunato's injuries because he needed him for business purposes. Meanwhile, Montresor's hatred keeps building, but he continues to act obsequiously while intending to take his revenge when the time is ripe.
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued, as was my wont to smile in his face, and he did not perceive that my smile now was at the thought of his immolation.
It is interesting to note that even when Montresor has his "friend" chained to the granite wall, he does not gloat or vent his hate and anger. He maintains the same obsequious manner he has been exhibiting all along. Fortunato will understand that Montresor has been deceiving him all these years.
“Pass your hand,” I said, “over the wall; you cannot help feeling the nitre. Indeed, it is very damp. Once more let me implore you to return. No? Then I must positively leave you. But I must first render you all the little attentions in my power.”