Poe's raven becomes a symbol of mourning and never-ending remembrance because he has been taught the single word "Nevermore" which acts as a reflection of the speaker's grieving mind that mourns the loss of his love as well as the melancholy of his soul.
According to Arthur Quinn, a Poe biographer, the propensity of man to seek the interpretation of signs is "one of the most profound impulses of human nature." In this nightmarish narrative of a melancholy and fixated mind, the raven becomes a symbol of the desolate mourning and loneliness of his heart that he initially embraces as he interrogates the raven. For, the speaker realizes that the answer will be the same each time since the bird knows only one word: the fatalistic "Nevermore." And, near the end of the poem after his repeated interrogation of this fatalistic symbol that reminds him of his grieving remembrance, the speaker finally attempts a break from his obsessive grief, imploring the raven to depart--
Leave my loneliness unbroken!--quit the bust above my door!
Take thy beak from out of my heart, and take they form from off my door! (100-101)
But, the raven refuses; instead, he answers with "Nevermore!" Now, the bird is no longer just a reflector of the speaker's obsessive grief; instead, he acts as a horrifying prophecy of mourning and a torturing remembrance that will endure forever.
And the Raven...still is sitting, still is sitting....
And my soul from out that shadow...
Shall be lifted--nevermore! (103-108)
--Additional Source:Quinn, Arthur Hobson. "Edgar Allan Poe, A Critical Biography." Baltimore: John Hopkins University Press, 1998 (second printing).