Nature reigns supreme over book learning in "if everything happens that can't be done." Birds and buds both have qualities that books simply can't match. The poem talks about teachers, about being indoors, but it's most dominated by the type of language and actions – running, leaping, flying – you'd find outdoors during, say, recess. The world, especially the natural world, becomes an amazing, joyous place in this poem, and it's the perfect place to fall in love.
Nature, in this poem, is presented as superior to books in terms of learning because nature gets closer to the truth of things.
Nature is used in this poem to express the exhilaration of love, because nature is full of the life and joy that a person feels when he is in love.