The narrator, Montresor, is extremely pleased to meet Fortunato in the street because, tonight, he plans to exact his revenge on Fortunato for "The thousand injuries" he had done Montresor. Montresor has come up with a plan and ensured that none of his servants will be home to witness his crime; all that remains is to find Fortunato and lure him into the trap Montresor has set.
This trap relies on Fortunato's one "weak point," his pride, especially the pride he feels in his taste and discernment as a connoisseur of wine. Montresor, when he sees his nemesis, tells him that he has purchased a pipe (126 gallons) of Amontillado (a dry Spanish sherry) and that he was "silly enough to pay the full Amontillado price without consulting [Fortunato] in the matter." In other words, he appeals to Fortunato's pride, thus increasing the likelihood that Fortunato will want to help him.
Montresor goes on to flatter him even more, saying that he will seek out another local wine expert to confirm his purchase because "some fools will have it that his taste is a match for [Fortunato's]." By calling those people "fools," Montresor implies that he knows that Fortunato's taste is better than this other man's, but by suggesting that he will ask for his assistance any way, Montresor guarantees that Fortunato will want to come in order to prove that his taste is, in fact, better.
Casually "bumping into" Fortunato in the street is the first step to exacting the revenge Montresor has so long desired.