The central theme of this famous poem is the profound beauty of nature and the pain that is evoked when Keats thinks about his own mortality and the way that he will have to lave it. Keats manages to capture feelings that are therefore part of the universal human experience and, when he describes the nightingale as a "light-winged Dryad of the trees" singing in "full-throated ease," he is able to evoke the beauty of nature through the words that he chooses.
You might find it interesting to consider some autobiographical details of Keats in relation to this poem. He wrote it when he was experiencing great mental turmoil due to the death of his brother from tuberculosis. Keats was beginning to suspect that he had the same disease, and he was also facing other personal difficulties. We can therefore understand the longing for death that is expressed in these lines:
I have been half in love with easeful Death,
Called him soft names in many a mused rhyme,
to take into the air my quiet breath...
This helps give this poem its rather bittersweet tone, as it combines and juxtaposes the beauty of nature and ideas of the eternal with the grim realities of decay and death that are faced by humanity. Whilst Keats is enraptured by his vision of the nightingale and how it represents eternal beauty, at the same time, at the end of the poem, he is forced to return to earth from his reverie to embrace precisely the harshness of life that his meditation on the nightingale has enabled him to escape. Although his reverie has allowed Keats to transcend his earthly and mortal limitations, this is only temporary, and perhaps the final question posed in the poem, "Do I wake or sleep?", indicates the way in which Keats finds the return to reality so challenging.