Why is the narrator afraid to answer the door when he hears tapping in "The Raven"?

The narrator is alone at midnight on a bleak December
night, and he has been reading quaint and curious forgotten lore. When he suddenly hears
a tapping, it naturally frightens him. He is also somewhat unnerved by the rustling of
his purple curtains. There is very little light inside his room. Most of it is coming
from the dying embers of his fireplace and from "lamp-light" by which he has presumably
been reading. What really frightens him is his own imagination. He imagines that it
might be a ghost outside, the ghost of his deceased loved one Lenore. When he stands
peering into the outer darkness without finding any human being there, he frightens
himself still further by "dreaming dreams no mortal ever dared to dream before." These
dreams, or fantasies, have to do with the possibility that Lenore, though dead, has come
back to visit him in answer to his prayers. He whispers the question, "Lenore?" He must
be feeling more and more certain that he is being visited by a supernatural spirit. He
is eventually relieved to discover that the tapping was only made by a bird which
apparently was a tame pet that had somehow gotten free and was seekiing shelter in
another human habitation.