"La Belle Dame Sans Merci" is John Keats's version of a medieval romance. It's about a knight who falls in love with a beautiful fairy lady. We immediately know that love is going to be a major theme. But where does love take us in the world of "La Belle Dame"? Not to weddings and happily ever after, that's for sure. "Love," in this poem, could be synonymous with "obsession." It's not a pretty thing.
The relationship between the lady and the knight in "La Belle Dame Sans Merci" is doomed from the start because the two are from fundamentally different worlds.
Love fails in "La Belle Dame Sans Merci" because it is one-sided: the lady "loves" in line 19, and confesses her love in line 28, while the knight only admires her beauty.