The speaker has fallen asleep dreaming about his lover, Lenore. He is depressed, and mourning, showing conflict.
A Raven comes to the door during the nap, and tells the speaker the truth that Lenore shan't return to him, hence "...'Nevermore.'"(Poe, end o' each stanza after 8). This leads to anger in the speaker due to his refusal to accept the fact, calling the bird a demon as the messenger of the unwanted.
After refusing to accept, the speaker finally accepts the truth, is lifted, to never be saddened by this again.