Shelley is comparing the skylark's experience of joy to the way humans experience joy. Shelley imagines that the skylark is a spiritual being. The bird sings only while flying and flies so high that he can't see the bird, so when he hears the skylark's song, it's as if it is an immaterial or spiritual voice of nature. The speaker (Shelley) imagines that the skylark knows only absolute joy: 

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. (76-80)

Shelley pines to know what such absolute joy is like. Humans can only imagine an existence full of bliss, completely without pain. He notes that his (humanity's) most sincere laughter and happiness are fraught with pain. In other words, we experience joy, but we also experience pain. In fact, he intimates that we might need this opposition to even know what joy and pain are. To know joy, we must also know pain. 

If humans did not know/experience pain and sadness, they might not understand or fully appreciate the joy of the skylark's nature or joy in general. In the next stanza, he says this directly: 

Yet if we could scorn
Hate and pride and fear;
If we were things born
Not to shed a tear,
I know not how thy joy we ever should come near. (91-95)