I would argue that the central idea of John Keat's poem “La Belle Dame Sans Merci” is the difference between appearance and reality and the consequences of mixing them up.
The knight meets a fair lady, “a faery's child,” who is beautiful, and he falls deeply in love with her. But there is much more to this lady than the knight realizes. Everything seems wonderful to the knight as he pursues his love. He makes garlands for the lady's head as well as bracelets and a belt, and she accepts him with apparent love. He places her on his horse and listens to her song. He eats the food she gives him and hears her proclaim her true love. He goes with her to the “Elfin grot” and kisses her as she weeps. Then he sleeps and dreams a horrible dream and wakes up into reality.
Now all appearances have fled, and the knight finds himself on a cold hill, caught up in the lady's spell with other kings and princes and warriors. Like them, he, too, is pale and lonely, now facing the reality of his love for the lady.
The lady is not at all what she appears to be, and she has given the knight some clues that he might have used to discern the reality beneath the appearance. Her eyes are unusually “wild.” She moans as she looks at him. He takes the moan for love, but it was not. Rather, it may indicate her desire to bewitch him. Her faery's song seems innocent, but it probably is winding a spell around the knight. The language in which she declares her love for the knight is “strange.” She weeps and sighs, perhaps in regret that she must turn this knight into a pale shadow of himself, yet she does it anyway. Indeed, this lady is not at all what she seems. Beneath her beauty and charm, she is sinister in reality, ready to capture the knight in her spells and entrap him on the “cold hill side” to which she has led many others.
The knight has failed to discern this reality beneath the lady's fair appearance, and the consequences of his error in judgment—his mistaking appearance for reality—are dire indeed.