In a nutshell, "La Belle Dame Sans Merci" is about being abandoned by the one you love. The knight gets abandoned and left on a "cold hill's side," even though he appears to be at death's door. The beautiful fairy lady, we know from the title, is "sans merci," or merciless. She abandons him without pity, and the knight's solitude becomes the framing image of the poem.
Questions About Abandonment
Why does the outside world seem to reflect the inner, emotional state of the knight? When he's in love, there are lots of flowers, and after he's been abandoned, the world seems desolate. Does the world only seem desolate in the first three stanzas because the knight has been abandoned?
Why does the lady abandon the knight while he's asleep? Can't she break up with him face-to-face?
Has the lady abandoned all of the "pale kings," "princes," and "pale warriors" from stanza 10? Are they just her ex-lovers, or are they dead? Was she responsible for their death?
Chew on This
The lady's abandonment of the knight is emphasized as an important theme in "La Belle Dame Sans Merci" through the repetition of the word "alone" in the first and final stanzas.