Romantic poetry tends to show a strong appreciation for the power of nature on humans and focuses on the emotional impact of experiences. It also tends to fondly recall the ideas of the past, appreciate the power of the human imagination, and respect innocence.
"La Belle Dame Sans Merci" draws on the Romantic idea the importance of the past. Keats is using one of the oldest forms of poetry, the ballad, to tell the story of the knight and the lady. The ballad opens with the description of the lonely knight -- evoking a somber sad mood as it describes the passing of the season into fall -- a stock metaphor for the middle to late stage of life. Once he meets the beautiful lady, the mood and images of the poem become more positive with references to flowers and beauty. Nature, and the love he feels for the lady seem to restore him spiritually and emotionally.
Once the knight is lulled to sleep he tells of his dream -- his imagaintion at work. Unfortunately his dream is a nightmare of deathy pale men. When he awakes, he is alone, exactly where he started. The positive emotion of the experience is gone, leaving him with only the memory of the lady who came, but without mercy, left him.
Keats's "La Belle Dame Sans Merci" has several elements of Romanticism. First, there is an element of fairy tales, as the narrator meets a fairy-like woman in the fields. His ride with her has the feel of a fairy tale, as it is atmospheric and dreamlike, and they travel by horse to an elf's grotto. The fairy tale becomes somewhat eerie and supernatural, which is another element of Romanticism, as the narrator falls asleep on the hillside and dreams of princes and princesses, who warn him about the fairy woman he has fallen in love with. The use of dreams and the experience of somewhat supernatural states are present in Romantic literature, as is the sense that nature is imbued with a mysterious element. The fairy woman herself seems to arise from nature, as she lives in the mead and finds wild roots and manna for the narrator. At the end of the poem, the narrator feels alienated and forlorn, another sentiment often expressed in Romantic literature.