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 composed this poem. He was inspired by Nightingale's song and completed the poem within one day. The poet begins the poem in a melancholic mood and disparagingly expresses how his “heart aches” as if he is poisoned or is drugged. He felt as if he drank Lethe's  water and was drowsy. Then it is revealed that actually it is the nearby bird, the Nightingale's song which is the reason of the poet's happiness. The poet addresses the Nightingale that he is happy in the bird's happiness.