At some point in our lives, we all come to grips with our mortality. Poe only illuminates those thoughts, feelings and fears in each of us. He does this through his masterful creation of suspense, as well as the singular focus of all of his short stories.
If you consider "The Tell-Tale Heart," the first of these elements is used masterfully. There is arguably no other story in which Poe so fully encompasses his readers in the tension and suspense, as readers can practically hear the beating beneath the floorboards for themselves.
In "The Masque of the Red Death," Poe takes the other element, the singular focus, and crafts an entire story around the idea of excess and selfishness.