Our sweetest songs are those that tell of saddest thought. (90)
If the bird song is pure joy, then human songs are always a mix, a combination of the sweet and the sad. In a way, we think that's part of what's wonderful about human life—you have to know the bitter to understand the sweet—but we can also see why it's bumming the speaker out. All he can think about is the distance between his song (poem) and the bird's.
Hate, and pride, and fear; (92)
Just a little more ugly, bad, sad human stuff. The world of the skylark (at least in this poem) is pure, free from any of the terrible feelings that make human life hard. We, on the other hand, are always tormented by these useless and painful emotions. Man, now we kind of wish we could be skylarks, too.