Stanza 8 is where I would start to look for this answer. In this stanza, the speaker seems to be mostly amused by his unexpected avian visitor. In his happier mood, the speaker takes it upon himself to ask the name of the raven. It's a perfectly understandable reaction. People always ask my dog her name instead of asking me her name. The speaker in the poem does the same thing.
"Tell me what thy lordly name is on the Night’s Plutonian shore!”
Quoth the Raven “Nevermore.”
The speaker isn't honestly expecting any kind of answer, so he is quite shocked that the raven gives any answer at all, let alone an answer that sounds like a word. The speaker is shocked into marveling at the raven's plain speech.
Much I marvelled this ungainly fowl to hear discourse so plainly,
Despite the marvel, the narrator admits that the bird's answer doesn't make much sense. He even points out that no other human being has likely ever come across an animal with the name "Nevermore." Next, he assumes that the bird will fly away and be gone by morning. Unfortunately, this is when the raven choose to repeat its "name."