At the outset of the poem, the speaker is feeling a great deal of grief, sadness and loss at the passing of his beloved Lenore. This sadness he has tried to drown in books but he is still hoping for daybreak to relieve him at least of the oppressive darkness that mirrors his mood.
But as he finds the raven at the window and lets it in, he is intrigued and questions the meaning of this stately bird, inky black and imperious. At first he wonders if there is some relief to be had by the entrance of this new visitor, but as he continues to question it and the meaning of its consistent pronouncement of "nevermore," his thoughts grow darker and colder.
He eventually wonders aloud if this dark visitor is a type of prophecy, if in fact his hopes of seeing his beloved Nevermore are doomed as the bird croaks "nevermore." Along with these questions, his mood darkens and he grows more and more despondent as he loses hope as he also finds that he cannot rid himself of this dark visitor and the gloom he represents.
At the finish of the poem, the speaker is utterly despondent, devoid of hope and any positive emotion, his "conversation" with the Raven has driven him to the depths of despair.
Of course it does, because raven only repeats 'nevermore'.