Why is the opening line of the poem repeated in the second stanza in "La Belle Dame Sans Merci"?

The first two stanzas serve to set the scene for the story
that is narrated to us thereafter of the beautiful, mysterious and malign lady, and if
we read the first two stanzas, what we can see they do is to create a strong sense of
setting for us before this story is told. Note what function they
perform:

"Ah,
what can ail thee, Knight-at-arms,

Alone and palely
loitering?

The sedge has withered from the
lake,

And no birds
sing.

"Ah, what can ail thee,
Knight-at-arms,

So haggard and
woe-begone?

The squirrel's granary is
full,

And the harvest's
done."

Note the speech marks
that indicate somebody has come across the knight that looks so sick and is asking him
why on earth he is remaining in such wilderness when autumn is setting in and nature has
transformed where he is into a bleak, barren landscape--which perfectly reflects his own
character after his experience of the beautiful women that has so enchanted him. The
repetition of the first line in the second stanza thus reinforces that something is
seriously wrong with the knight, and serves to arouse our interest and anticipation in
finding out what precisely has happened to him.