What are the consequences of love as Keats describe them in "La Belle Dame Sans Merci"?

John Keats's "La Belle Dame sans Merci" is a Romantic
ballad that tells of the misfortune of a knight who falls in love with a maiden who is
without mercy--"sans merci."  Much like some ancient myths in which the fertility of the
land is connected to the health of a heroic figure such as a king or a knight, upon whom
a spell is cast and must be thrown off for the land to again be bountiful, Keats's poem
reflects this motif.

After the ethereal lady takes the
knight to her "elfin grot," in line 29, she lulls him to sleep; however, while he is
asleep, the knight has a horrifying dream in which he envisions "pale kings and princes"
who cry out to him that the lady without mercy has him entralled and, thus, in terrible
danger.  For, they, too, are the victims of the "faery child's" eternal enslavement. 
Frightened by this vision, the knight awakens only to find himself thus enslaved as he
wanders alone through the woods that now has withered in its
beauty:

And
this is why I sojourn here,

Alone and palely
loitering,

Though the sedge has withered from the
lake,

And no birds
sing.

In addition to his
enslavement to the maiden's caprice, the knight suffers from unrequited love as a
consequence of his meeting her and her abandonment.