"It must be understood", as Montresor might say, that he only appeared to be happy to meet Fortunato. In fact he was simply keeping up appearances to allay suspicion. However, we might be blunt and say that Montresor was happy to meet Fortunato because the first step in killing him was to find him.
Contrary to the way this question is phrased, Montresor is said to be greeted by Fortunato, and with excessive warmth, probably due to Fortunato being drunk, and to his personality being naturally a bit too much on the familiar side. Montresor keeps up his illusion of being happy to see Fortunato because he has always smiled in his presence, although Fortunato does not suspect that Montresor's smile is now inspired by the thought of Fortunato's "immolation" - meaning his burning in the fires of hell for his crimes.
Montresor is also using friendly language to manipulate Fortunato, by playing upon his ego. He needs to appear happy to see Fortunato, but ready to pass him by, so that Fortunato will see their meeting as one of a friendly opportunity to be had, and his own eagerness will further blind Fortunato to the danger that awaits him.