As the poem opens, the speaker is feel dejected and depressed. Even nature can't rouse him from this state. He gazes at the "yellow green" evening sky with a "blank" eye. As night falls and he sees the crescent mood and stars, he continues to feel despondent and can "see" but not "feel" their beauty.
The speaker goes on to contrast this state with the joy he has felt in the past. This joy is not merely happiness but the "strong music in the soul." It is the feeling that animates everything else and allows the speaker to experience emotionally, not just intellectually, the beauty all around him. This kind of joy is "light" and "glory." Nature itself is not truly experienced as beautiful until this soul joy makes it so: it is this deepest joy that has "beauty-making power."
This kind of joy can only be experienced by the pure and only when they are at their purest. It is a special kind of privilege that allows one to glimpse or imagine "a new Earth and new Heaven." It is from this joy that
flows all that charms or ear or sight,
All melodies the echoes of that voice,
All colours a suffusion from that light.
In a way, Coleridge is challenging the idea of his friend William Wordsworth that nature can bring joy and solace to the soul with its beauty, harmony, and peace. Instead, Coleridge argues that it is a certain sublime joy that first suffuses the soul that allows a person to experience the beauty of nature. It is not from the outside in that we experience joy and beauty, but from the inside out. When we experience inward joy, this radiates out to make the universe beautiful to us.