How does the author contrast the life of the bird and the man in the poem "To a Skylark"?

In "To a Skylark," Percy Bysshe Shelley sets up a stark contrast between the life of a bird and the life of mankind. For the first half of the poem, the narrator focuses on the skylark itself, using simile and personification to present a sense of wonder at the sight of the bird. Midway through the poem, Shelley begins to contrast the experience of the bird with that of the narrator, saying:

Teach us, Sprite or Bird,
What sweet thoughts are thine:
I have never heard
Praise of love or wine
That panted forth a flood of rapture so divine.

Chorus Hymeneal,
Or triumphal chant,
Match'd with think would be all
But an empty vaunt,
A thing wherein we feel there is some hidden want. (61–70)

Here Shelley draws a clear distinction between the bird and man. To Shelley, the "sweet thoughts" of the bird are far more rapturous and "divine" than those of man (60). Further, even the greatest earthly choirs are "but an empty vaunt" when compared with the song of the skylark (69).
Shelley goes on to compare the double-edged sword of human experience with that of the skylark:

With thy clear keen joyance
Languor cannot be:
Shadow of annoyance
Never came near thee:
Thou lovest: but ne'er knew love's sad satiety.

Waking or asleep,
Thou of death must deem
Things more true and deep
Than we mortals dream,
Or how could thy notes flow in such a crystal stream?

We look before and after,
And pine for what is not:
Our sincerest laughter
With some pain is fraught;
Our sweetest songs are those that tell of saddest thought. (76–90)

For humans, love and laughter are always tinged with pain. Even the happiest experience will eventually be darkened by the specter of loss because of our understanding of life and death and our compulsion to "look before and after," rather than simply live in the moment (86). The speaker asserts that humans desire what they don't have; they lament the things they have lost, and human dreams are limited in their scope. However, for the skylark, love is simply love; the moment is the experience; there is no understanding of eventual loss. Because of this, their dreams are more "true and deep" that those of humans (83).Shelley ends the poem yearning for "half the gladness" that the skylark knows. The contrast between bird and human is bittersweet. While we humans have a more nuanced understanding of life, the particulars of our understanding prevent us from experiencing life as completely as the skylark.