Keats in his "Ode to a Nightingale" reacts to the happiness brought to him by the song of the nightingale. Keats focuses directly on his own feelings and on the way the song of the nightingale has alleviated certain emotions and concerns he has been experiencing:
My heart aches, and a drowsy numbness pains
My sense, as though of hemlock I had drunk,
Or emptied some dull opiate to the drains (1-3).
Keats shows here how the song of the nightingale actually brings him to life, generating the poetic spirit in him and causing him to put his thoughts down on paper. The imagery he selects furthers the sense of contemplation and self-analysis on the part of the poet and creates a contrast between the song of the nightingale, a song of life, and the poet's contemplation of death.
Keats uses iambic pentameter for the opening of his ode and then varies the meter according to the mood and the meaning in later stanzas. For Keats, his contemplation of the song of the nightingale is an occasion for him to escape from the world of reality. Keats sees the song as evidence of the world of imagination, and this is the world of the poet as well. Keats can lose himself in the song of the nightingale. To a degree, this is a mini-death or a preview of death for the poet, and he considers the meaning of death and what sort of oblivion it would bring. The song of the nightingale causes the poet to want to leave the world of reality and enter the world of the imagination:
Not charioted by Bacchus and his pards,
Though the dull brain perplexes and retards (31-34).
Keats here notes that his escape will not be through drink but through poetry and the imagination, though his human brain may be an obstacle to be overcome as he must shift from the real world to the poetic world of the nightingale.
Yet death beckons as well and would free the poet from the confines of this world