To simplify a complex question and using "A Thing of Beauty" (from Endymion) as a starting point, Keats expresses the idea that beauty is that in life which brushes "away the pall / From our dark spirits" (a pall is a shroud). Our spirits are darkened ("our dark spirits"), according to the poem, by despondence, lack of nobility in people's human natures, gloominess, unhealthfulness, and general "o'er-darkn'd ways."
Keats writes that the sources of this pall-removing beauty are such things as "the sun, the moon," trees, a "shady boon" (which is blessing or benefit), daffodils, green world, "clear rills" (brooks or rivulets), "cooling covert" (a hiding place), a bushy overgrowth in the midst of a forest sprinkled with "musk-rose blooms." He goes so far as to say that the after-life fate we conceive for the mighty who have died, which is to be "An endless fountain... / Pouring unto us...", is part of the beauty that "moves away" our pall.
In "La Belle Dame Sans Merci," we see accord with the above discussion. In one opinion, the poem is a lesson against the deceptive power of feminine beauty and, in that regard, can be compared to Tennyson's similar warning against masculine beauty in "The Lady of Shalott." Another view sees the poem as Keats' lament at the love-betrayal of life since he knew his life was soon to end in tuberculosis.
In the first view, while the lady, representing feminine beauty, is the betrayer, it is nature that is all around giving great gifts to the couple while they idle in the meadow or amble on their horse ride to her "grot" (a grotto, which is a cave). Even after the knight's death, it is a lily on his forehead that is the parting tribute from beauty to his lost life. In the second view, life is the betrayer and is cast as a faery lady while nature is still that which gives joyous moments and comfort, as through the lily on his brow.
In addition, the poem accords with the above because, in the first view, it is the lady's lack of nobility in her nature and "o'er darkn'd ways" that cause the knight's morbid death. In the second view, it is life's despondence and unhealthfulness that cause the knight's death. Again, in both views it is the lily that gives the parting, lingering kiss of benediction to the departed knight.