Question

Why does the solitary reaper's song seem to be never ending?

Answer 1

The poet finds the maiden's song so melodious, even though he really does not understand what she is singing about, to the extent that it sticks to his heart. He was so taken into the moment by the nature of the beauty of the song that it seemed an eternity's time listening to the song. Even after he has gone up hill and hears the song no more, he still carries the melody in his heart.