Why does Montresor not like Fortunato?
The text is being narrated by Montresor himself, but the reader does not realize that until toward the end of the story. Sadly though, Montresor never gives the reader any specifics about why he does not like Fortunato. The opening lines of the story indicate that the two men are more than mere acquaintances.
THE THOUSAND INJURIES of Fortunato I had borne as I best could, . . .
A "thousand injuries" is likely hyperbole, but only using 10% of that number is 100. I have never been injured in any way 100 times by a mere acquaintance. When the men meet during the festival, Montresor greets Fortunato in a very friendly manner, so it's clear that Fortunato doesn't consider Montresor an enemy.
I said to him—“My dear Fortunato, you are luckily met. How remarkably well you are looking to-day. But I have received a pipe of what passes for Amontillado, and I have my doubts.”
Going back to the opening line, Montresor admits his reason for wanting to kill Fortunato.
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
Montresor apparently doesn't have any problems with being hurt 1000 times, but he takes grave offense to being insulted a single time. That's why Montresor doesn't like Fortunato. The reader never finds out what the insult was though.