Some elements of Romanticism:
a reaction against rationalism and efficiency
a return to the natural
an emphasis on emotion, and irrationality, rather than rationality.
To see why Shelley is a Romantic poet take a look at these elements in Shelley’s Ode to a Skylark.
While odes are usually to great men or creations of men, this ode praises something natural, a bird. Moreover, the bird is praised for its “unpremeditated art.” That is the bird didn’t plan or think to make its song, it just poured out its emotions. And the person looking at the bird doesn’t try to analyze it: “what thou art we know not”
Instead what is appreciated is “shrill delight,” and “harmonious madness.”
Readers are asked not to analyze but to listen to the bird.