These lines aren't joyous in the slightest. The poetry is just an ideal escapist way to deal with the pain of human existence where Keats has used the idea of negative capability— negating the need for definitive answers and embracing uncertainty and ambiguity. While the nightingale's song represents beauty and an escape from suffering, the speaker's response is rooted in sadness and longing, not happiness.
Today your soul is feeling doubtful about the feeling of some poetry or poetry,@InkyWhispers . Is there something special that is making your heart feel this joy?