The speaker in Shelley's poem hears nothing but joy reflected in the skylark's song. He contrasts that to human emotions, which are colored by pain, and human music in which the
sweetest songs are those that tell of saddest thought.
Our beauty is always mingled with sadness, while the skylark is wholly happy.
Further, the skylark lives eternally in the present moment and is content with that. In contrast, we humans are always looking to the past and the future ("before and after") and "pining" for what we cannot have. We are not perfectly in harmony with the cosmos the way the skylark is.
Although he is a poet himself, Shelley decides the skylark has in its heart a more beautiful song than any human poet can write. Like the Romantic he is, Shelley exalts nature over civilization and finds in the songbird's music more treasure than in the words humans write:
Better than all measures
Of delightful sound,
Better than all treasures
That in books are found.
The poem is also Romantic in being lyrical in its love of nature. Shelley wants us to feel the deep emotions he experiences at the sound of the bird's song so that we, too, will stop and pay attention to its beauty. Perhaps, as well, if we dwell on the contentment and joy of the skylark, we will find such peace and happiness in our own human hearts.