I agree that Shelley is amazing. The attention to nature and the personification which exists is beautiful. Also, as litteacher points out, the poem is a metaphor. The use of natural imagery allows engaged readers to dig deeper for the underlying meaning.
We look before and after,
And pine for what is not;
Our sincerest laughter
With some pain is fraught;
our sweetest songs are those that tell of saddest thought.
Here in the poem the life of Skylark has been contrasted to the life of human beings. Human beings, whether they look back to their past or they look forward to their future, feel intense desire for what they have not. Therefore the human life is full of disappointment and frusstrations. There is an element of pain mingled even with their most genuine laughter. They can never enjoy unadulterated happiness. The sweetest songs of human beings are those that are full of sorrows and grief.
On the contrary, the songs of the skylark are an expression of pure joy. Because it soars up higher and higher into the sky and hardly pays ant attention towards earthly cares and enjoyments. The more the bird flies high, the more it comes nearer to the Heaven. From there the bird pours fourth its melodius strain to feel the earth.