One of the most prominent features in Romantic poetry is the use of the creative imagination. In the Romantic movement, there was a large emphasis on the individual and contemplation of nature. Keats began composing "Ode to a Nightingale" in the spring of 1819, sitting under a plum tree while listening to a nightingale. The Romantic motif expressed in this poem is the self/individual seeking a connection to nature through imaginings, dreams, and visions.
In the first two stanzas, the speaker imagines and wishes for a drug or alcohol. He wants to be released from the burden of human worries, to forget them ("Lethe - wards") so that he might be as free as the nigtingale: able to "Singest of summer in full-throated ease." To the speaker, the nightingale is literally and figuratively "in tune" with nature, seemingly without a care in the world.
Meanwhile, the speaker is consumed with care. In the third stanza, Keats makes a reference to his brother, Tom, who died the previous year.
Where palsy shakes a few, sad, last gray hairs,
Where youth grows pale, and spectre-thin, and dies;
Where but to think is to be full of sorrow
The speaker switches from a desire to be drunk on forgetfulness to being inspired by the poetic ("Poesy") imagination. Here is the connection between poetic imagination in connection with nature, a common Romantic motif. In the last two stanzas, the speaker notes that the nightingale "was not born for death." He comes out of his poetic/imaginative revery, its spell ("fancy") is broken at the end of the poem. The speaker conflates (combines) this idea of the seemingly immortal nightingale ("not born for death") with the power of his poetic imagination, poetry itself. This is if to say that imagination, or creation in all its senses, has the potential to be like the immortal song of the nightingale. The song itself, like a poem, stands outside of time, as if the singer/speaker has no concern for mortality and therefore is in tune with nature singing with complete ease. This has been the speaker's goal in the poem.