What love of thine own kind? what ignorance of pain? (75)
If we wanted to explain the feelings in this poem to a chimp (and we do—alas, our Shmoop for Chimps™ idea never got off the ground), we'd do it like this: bird = happy, person = sad. At least that's how the speaker of the poem seems to see it. He imagines that only a creature who had never known pain could sing such a purely happy song. We're not sure what this guy thinks he knows about bird pain. Maybe a skylark has problems too. A mortgage on the nest? Three eggs about to hatch? Gotta teach the kids how to fly? Who knows?
Thou lovest: but ne'er knew love's sad satiety. (80)
Just like this bird isn't supposed to feel pain, he's not supposed to know the sad and bitter side of love either. That's what humans feel. The speaker is steadily reinforcing his big idea: that only a bird could really live without sadness. For a human, to be alive is to feel sadness. Even something as beautiful as love is unavoidably tainted with sadness. Cheerful thought, huh?
We look before and after, And pine for what is not: (86-87)
This, according to the speaker, is the reason why humans can't help but be sad. We're always thinking about what happened in the past and what's coming our way. We can't just feel the pure joy of the moment. It's kind of the tragic irony of this poem, too. The more the poet tries to live in this instant, to communicate the power of the bird's song, the more he realizes he'll never be able to. He's human, and so he's left to feel bad about what he can't have, to "pine for what is not."