This is the second of a whole string of stanzas that focus on a single simile and really dig into it. In this case, the bird's song is compared to a "high-born maiden" in a tower. Ever seen Sleeping Beauty? That's exactly what we're talking about. We're in serious Disney princess territory here!
Line 41: This lonely maiden sings to make herself feel better. Again, the speaker is trying to connect the skylark to human feelings and human experiences. Even though this bird probably isn't in love, a simile like this allows the speaker to connect it to all of those exciting, romantic feelings that we find in fairy tales.