" The final line of the ballad, "And no birds sing" reinforces the idea of loneliness and emptiness, and creates an invisible link with the beginning of the poem, more precisely the first stanza which ends with the same line. At a closer reading, one notices that the roles of the knight and the lady change throughout the following stanzas, with each of them being successively dominant over the other. In stanzas IV-VI, the first two lines focus on the knight who is clearly in control -- "I met," "I made," "I set her" -- the use of the first person pronoun is a clear indication as far as the power relations in the poem, whereas lines 3 and 4 refer to the actions of the lady. Moreover, stanza VII is completely devoted to her with verbs such as "she found" and "she said." The following stanza grants the lady the dominant position as far as the narrative level of the ballad: "she took me" and "she wept and sigh'd." This power struggle expressed through pronouns is actually very relevant to the task of understanding how this mysterious woman enters and ultimately changes the knight's life. In the beginning, the audience sees a depressed and lonely knight whose anguish is also expressed through the use of setting imagery: "the sedge has withered from the lake," "the harvest's done" and "fading rose." These images suggest that the knight is feeling sad and lonely after his meeting with the Belle Dame. However, one could argue that the knight was feeling depressed...

In fact, it could have been this depression and inner void that determined the knight to escape to the world of imagination where he is able to create a world according to his needs and desires, a world where his dreams can come true. This is why he imagines an encounter with a beautiful woman who shares his feelings, and gives him the sense of worth and pride that he so desperately lacks in real life. Moreover, in his imagination the "pale" knight feels more powerful and in control of his own life.
There are several clues which point to the theory that the knight had in fact imagined this encounter, such as the repetition of the word "faery," "the elfin grot" and the lady's eerie song. Nonetheless, even in his imagination he starts to lose power and head towards unhappiness as his imaginary world seems to be slowly collapsing in front of his eyes: "I saw pale kings and princes too, / Pale warriors, death-pale were they all; / They cried - 'La Belle Dame sans Merci / Hath thee in thrall!'" in fact, the kings, princes and warriors are all avatars of the real world pointing at the fact that one cannot escape their problems by living solely in our imagination. He awakes and realizes that it had all been a dream. Depression sets in again as the knight is unable to seize control over his own life: "And I awoke and found me here / on the cold hill's side."

Keats, John. La Belle Dame sans Merci.