The principal theme in the poem is mortality.
Keats was a morbid person and was more than normally morose because of the recent death of his brother Tom from a wasting illness (Tuberculosis). He was right to feel apprehensive as he himself had caught it and died the follwing year.
In the poem he imagines dying from a lethal drink and joining the immortal nightinale in the trees, totally ignorant and free of life's cares and woes, sickness and death. He contrasts the mortalilty of individual men with the immortality of birdsong, of which the Nightingale is the leading exponent.