The speaker, listening to the beautiful song of the nightingale, wants to forget all the problems that go with human consciousness. Keats would like to fly away and be amid the beautiful flowers with the nightingale. He wants to forget about such situations as stress, disease, old age and despair. From his perspective, the nightingale does not ever experience anxiety about such human problems as:
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
Although the speaker has mixed feelings about death—he says he is "half in love with easeful Death" and feels it would be lovely to die while listening to the intoxicating nightingale's song—he nevertheless contrasts the human consciousness of mortality to the nightingale's oblivion as it lives in the present moment. He calls it an "immortal" bird because it stays the same throughout history. It lives just as it did in Biblical days, singing the same sweet song without forethought about the future.
To become a nightingale represents, to Keats, leaving behind the pain that human consciousness brings.