Basically, the speaker is playing out the fantasy he started in line 75, imagining that the skylark can make pure art without any hint of suffering.
When he hears the sound of that" clear" and "keen" (sharp, poignant) joy ("joyance"), he cannot imagine how it could ever be connected to sadness and depression ("languor").
Shadow of annoyance Never came near thee:
In the speaker's mind, this bird is so perfectly happy that it could never even feel the "shadow of annoyance."
Clearly, he's putting a lot of feeling onto this bird! We're pretty sure this is more about the speaker's problems than the bird, though.
(Really, we think it's probably pretty tough to be a bird. You always have to be hustling to find bugs to each, you have to watch out for hawks all the time and worry about your kids falling out of the nest. If a bird could get annoyed, that would probably be enough to do it!)
Thou lovest: but ne'er knew love's sad satiety.
The speaker imagines that the bird can feel love but not the sad "satiety" (the feeling of being filled with something) that comes with being full of love.
It's not really clear how he knows that a bird is capable of love. Basically, through this personification the skylark can be everything the speaker dreams of, without any of the pain. Since it can't talk back, he can fantasize about all the pure joy it must be feeling.