Through wood and stream and field and hill and Ocean A quickening life from the Earth's heart has burst As it has ever done, with change and motion, From the great morning of the world when first God dawn'd on Chaos; in its stream immers'd, The lamps of Heaven flash with a softer light; All baser things pant with life's sacred thirst; Diffuse themselves; and spend in love's delight, The beauty and the joy of their renewed might.
Things are going on as they have always gone on, since the beginning of time, the "great morning of the world."
Our speaker uses lots of energetic imagery here to explain the process of life as it is seen in nature.
Life is "burst[ing]" through the planet, quenching the thirst of every thing on earth, from God to "baser things," like plants and creatures.
This stream of "life" flows like water, and when any being drinks of it, they are filled with "love's delight" and feel renewed.
Why is the speaker establishing this "stream of life?" Perhaps he is trying to contrast the joy of being alive to the lack of joy Adonais-Keats will experience in death. In a way, he's trying to make us think about what the dead are missing out on so that we feel even more like mourning. So we guess we'll be turning this smile right back upside-down now.