Nature takes center stage in this poem in odd ways. Pretty much everything that Keats has to say about love and desire (and even about writing) comes out as a meditation on the natural world. Nature, you see, becomes a stand-in for everything that occurs in human life… which is funny, because when the speaker decides to reject all of the trials of human life, where does he turn? You guessed it… to nature. If you wanted to make sure that Keats was a Romantic poet, well, here's your proof.
The natural world is a source of comfort for the speaker. It calms his fears.