In "The Raven," the speaker is mourning the loss of his mistress, Lenore. The poem is about how this man deals with his grief. He attempts to distract himself by looking through a book ("volume of forgotten lore"). But he keeps dwelling on his grief. In this case, the suggestion is that some people tend to dwell on their grief to the point of self-torture. The raven represents man's inability to accept, forget, or deny his grief. He repeatedly asks the raven questions, knowing he will get the response, "Nevermore."
The man asks for the raven's name. The response is "nevermore." He asks the raven (symbolizing his grief) if it will leave. The raven gives the same response. The man can not (or will not) escape his grief. I say "will not" because the man continues to ask these questions, knowing that he will get the same negative response.
The speaker asks if he will ever be able to forget his grief:
Respite—respite and nepenthe from thy memories of Lenore;Quaff, oh quaff this kind nepenthe and forget this lost Lenore!”Quoth the Raven “Nevermore.”
Tell this soul with sorrow laden if, within the distant Aidenn,It shall clasp a sainted maiden whom the angels name Lenore—Clasp a rare and radiant maiden whom the angels name Lenore.”Quoth the Raven “Nevermore.”