What is the most plausible theme in The Raven?

The most plausible theme in the poem "The Raven" by Edgar Allan Poe is loss and the resulting grief and madness that can consume an individual. The speaker, overwhelmed by the loss of his beloved Lenore, becomes obsessed with the raven's constant repetition of the word "Nevermore." This one-word response reflects the speaker's own despair and hopelessness, and the raven's presence serves to amplify the speaker's grief and eventual descent into madness.