The reader can interpret as Cybil did that Montressor is still gloating about his success. Also, one can consider that if Montressor had committed the perfect crime, he would be itching to tell someone. It can be assumed that he hasn't told anyone since he does not explain that he is in jail. The fact that this memory is still so vivid for Montressor suggests that he has obssessed about it or at least recalled it several times since the actual event occurred. One could argue that guilt drives him to confess his success to the reader.
Rather than telling it in the future, Montresor tells the story in the form of a flashback, recreating the events that occurred fifty years before when he walled up Fortunato. Not until the end of the story do we discover that he committed the deed many years ago when he describes Fortunato's bones: "For the half of a century no mortal has disturbed them." Montresor has managed to get away with his crime for fifty years, and only now does he finally boast of his success.
I have discussed this in class, and my teacher had said that it was possible that Montresor was dying. He feels guilty, and needs to tell someone of his sinful deed. He is telling he reader exactly what he did. In the beginning of this story, Montresor states "You, who so well know the nature of my soul...". He is trying o get himself to believe that what he did was right. He wants to believe it, but he is finding it difficult to do so, so he is telling the reader what he did to get reassurance.