Almost everything we know about how Montresor feels about Fortunato in "The Cask of Amontillado" by Edgar Allan Poe can be found in the first paragraph of the story:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged; this was a point definitively settled--but the very definitiveness with which it was resolved precluded the idea of risk. I must not only punish, but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.
So here is what we know. Montresor feels wronged by Fortunato because of some insulting remark or action Fortunato made toward him. We do not know precisely what it is, but it is evidently much, much worse than a "thousand injuries" which he was able to endure.
We also know that Montresor intends to take revenge on Fortunato rather than forgive him, which he could just as easily do. This revenge will be the perfect kind of punishment, in which Fortunato will know exactly who is punishing him and why, and for which Montresor will remain unpunished.
One other thing we know about how Montresor feels about Fortunato is evident in the method he chooses to lure Fortunato to his home during Carnival season. Montresor clearly thinks Fortunato is too vain, for he uses that excessive pride to create a scenario where Fortunato is coming to check out Montresor's unlikely purchase of Amontillado because he is a better judge than Luchesi. In this Montresor may be right, since the strategy works. Though it is clear that Montresor is not completely sane, he does seem to understand human nature, at least a little. Not only does his ruse work on Fortunato, but Montresor's trick to clear the servants out of his house worked because he knew their natures, as well. This does not mean, of course, that we can accept Montresor's judgment in all things.
Whatever the insult, it must have been awful for Montresor to go to such elaborate lengths to avenge himself--or else Montresor is not sane and has created a mountain out of a molehill, so to speak. The evidence suggests the latter, since Fortunato does not seem to have any ill feelings at all toward Montresor; in fact, even at the end Fortunato does not seem to understand what could be prompting Montresor's odd (and deadly) actions. It is likely that the relationship between them was friendly enough, but one day something in Montresor just snapped and he allowed himself to be wounded by some careless remark by Fortunato.