What might be the circumstances in which the narrator, Montresor, decided to reveal the story of "The Cask of Amontillado"?
One of the most commonly accepted interpretations of this question is that Montresor is facing his own death. There are many hints to this, both at the beginning and the end. Keep in mind, before I give the clues, that Montresor is an adult man when the scene with Fortunato occurs - adult enough to be master on his own home and servants.
Already an adult, Montresor tells us in the last paragraph, that the event he has described came 50 years before:
For the half of a century no mortal has disturbed them. In pace requiescat!
This would make Montresor somewhere between 75 and 100 years old - perhaps sick and facing his own death and needing to tell his story.
Now understanding how old he is, we go back over the story and find that Montresor was addressing someone who knew his soul:
"you who so well know the nature of my soul"
Typically, the soul is the purvey of a religious scholar. The assumption is that Montresor, facing death, is confessing either to a priest or to God directly in the hopes of absolving himself of his sin before he passes on.