How is "Ode to a Nightingale" a Romantic poem?

"Ode to a Nightingale" is a Romantic poem in that it expresses a deep connection between the creative artist and nature. For Romantics like Keats, the natural world isn't just a collection of pretty objects; it is a living, breathing force in its own right, a vast organic whole of which we as humans are an intrinsic component.
In the poem, Keats tries to establish his connection to the natural world through the imagination, an especially vital faculty for Romantic artists. The imagination formed an essential part of the Romantic approach to life and art. It was seen as providing the artist with a means of penetrating to the deepest truths of existence. And it is through the imagination that Keats seeks to establish a connection with the nightingale, whose timeless sweet song has endured throughout history and will live on into eternity.
In the song of the nightingale we see another Romantic theme—that of the primacy of art over life. Historical epochs come and go, successive generations eventually die out, but art, as epitomized by the nightingale's song, will live forever, as it partakes of the eternal.
https://www.poetryfoundation.org/poems/44479/ode-to-a-nightingale