In "The Raven" by Edgar Allen Poe, the speaker is struggling to deal with the loss of Lenore, his love, who has died. The speaker is holding himself together before the raven arrives. The bird's continuous and negative response to the speaker's inquiries (of "Nevermore") drives the speaker to a desperate mental state.
…vainly I had sought to borrow
From my books surcease of sorrow—sorrow for the lost Lenore—
For the rare and radiant maiden whom the angels name Lenore—
Nameless here for evermore.
The reader has submersed himself in reading to keep his mind off of the loss of Lenore. Soon there is a "rapping" and "tapping" on the door, with no one there; but at the window, a raven enters as the speaker checks to see what is making a new tapping. At first the speaker is amused by the bird's presence, but as he continues to speak to the bird, he becomes more introspective and more depressed. The speaker morosely observes that the bird will surely leave him the next day, just as his friends have abandoned him.
Nothing further then he uttered—not a feather then he fluttered—
Till I scarcely more than muttered, “other friends have flown before—
On the morrow he will leave me, as my hopes have flown before.”
Then the bird said, “Nevermore.”
The speaker notes that this time the bird's response actually fits with the speaker's comment: the bird says "Nevermore," meaning it will never leave. But the speaker observes that at some point, if the bird continues uttering the same thing, its response will eventually make sense to some question. Suddenly the speaker picks up a scent that reminds him of his lover and he pleads that he be released from the memory of Lenore; the raven notes that his will not happen. ("Nevermore.")
“Wretch,” I cried, “thy God hath lent thee—by these angels he hath sent thee
Quaff, oh quaff this kind nepenthe and forget this lost Lenore!”
Quoth the Raven, “Nevermore.”
The speaker begs the raven (that he believes God and the angels have sent to him) to tell him that he can hope to see his beloved in heaven, but the raven says this will never happen either. Distraught and angry, the speaker tells the bird to leave him, but again the bird repeats "Nevermore." As the poem ends, the bird is still with the speaker, on the bust of Pallas.
The bird's continuous responses create a continuous cycle in which the speaker must confront his loss and the realization that the cycle will never be broken. The bird's negative response, which never alters, develops a circular movement where the speaker is confronted over and over with his pain. At the end, he realizes that he has no hope of ever living a life that is any different than the empty one he is struggling through now. The raven's response of hopelessness mirrors the speaker's growing sense that his life will never be free of this cycle of pain—and loss (one of the poem's themes).
And my soul from out that shadow that lies floating on the floor
Shall be lifted—nevermore!
The Raven won't stop even though the speaker wants him to. It reflects the loss of control of his mind. The Raven is taking over.