How does the nightingale’s song plunge the poet into a state of ecstasy?

When Keats was sitting under a plum tree in the garden of his house, in Hampstead, he stroked with an idea to compose a poem called “Ode to a Nightingale.” He was actually inspired by the song that was sung by Nightingale and he was done with his poem within one day only. The poet began this poem keeping in mind the melodious voice of a nightingale, the mood of the poem was melancholic and initiated a poem by saying that how his “heart aches” as if he was poisoned. Song of Nightingale was making him happy and making him felt in peace as he was comparing with Lethe river water as he was feeling that he was alive and forgot that there is something called death as he was fully indulged in her song and was living that song.