In "The Fall of the House of Usher," why is it significant that the narrator is affected by Usher's condition?

The narrator in Edgar Poe's story The Fall of The House of Usher can be and has been seen by alot of commentators as a psychic counterpart of Usher. These psychic doubles of self and the Other are ever-present in Poe's fiction.

Seen from this angle, the narrator is like a foil to Usher's character and the whole story can then be seen as a horrific unification of the self and the Other whereby the self realizes its affinity with the Other and resultantly its own alterity. This is the crux of the kind of alienation that Poe produces in the story. The final efforts of the narrator are to flee away from the House, which is a horrid psychic space of this identification. In the fianal moments when Madeline comes back from the vault and dies, a strange madness, the seeds of which were always there in Usher's Melancholia, takes complete hold on him and it is at this point that the narrator feels horridly similar to him, in terms of the common state of extreme nervous tension, on the verge of going mad.