This excellent poem creates a division between the world in which Keats is forced to live, which is characterised by pain, suffering and death, and the world of the nightingale, which is seen by Keats as a symbol of the eternal and of beauty itself, which remains unsullied by what happens in the earth below. consider how this division is introduced in the third stanza of this poem:
Fade far away, dissolve, and quite forget
What thou among the leaves hast never known,
The weariness, the fever, and the fret
Here, where men sit and hear each other groan;
Where palsy shakes a few, sad, last gray hairs,
Where youth grows pale, and spectre-thin, and dies...
Note the way that the nightingale lives "far away" where it is able to forget the kind of sights and experiences it has never known. Life for Keats and humans at large is characterised by "weariness," "fever" and "fret," all leading to eventual death. By contrast, if we look at the fourth stanza, we can see that the world of the nightingale is described as a fantastical place of enchantment with the "Queen-Moon" on her throne and "Clustered around by all her starry Fays." In contrast to the life of humans, the nightingale "wast not born for death" and is an "immortal bird." It has never had the experience of "hungry generations" treading it down.