Please explain the following stanza from "Ode to a Nightingale" by Keats.
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 One minute past, and Lethe-wards had sunk:
'Tis not through envy of thy happy lot, 5
But being too happy in thine happiness,
That thou, light-wingèd Dryad of the trees,
In some melodious plot
Of beechen green, and shadows numberless,
Singest of summer in full-throated ease. 10
You have cited the first stanza of this remarkable poem. The stanza opens by describing the melancholy attitude of the speaker. We are not given a reason for his feelings, but perhaps he is heartbroken, as he describes "numbness" as something that characterises his condition. It is in this state, that the speaker hears the beautiful song of the nightingale. The quality of this song, as it is both exquisite and melancholy, seems to fit the mood of the speaker, as it captures the paradoxical response of both happiness and sadness. The speaker feels happy because of the way that the bird represents a lust for life as he sings in "full-throated ease," but at the same time it heightens his state of being alone and his melancholy thoughts. The first four lines are important because it helps explain the way that the speaker goes on to have a kind of out-of-body experience as the song of the nightingale enraptures him and moves him beyond himself.