What is the inner meaning of the lines "Our sweetest songs are those that tell of the saddest thoughts" written by Percy Bysshe Shelley?
Like many of the great Romantic poems, "To a Skylark" possesses a remarkable coherence, by which we mean that all of its parts relate to the whole. One can take a single line, such as this one, and see its relation to the whole poem's meaning.
Looking at the dramatic situation behind the poem, the speaker hears but does not see a skylark, and he contemplates the music this bird makes. The long lines themselves seem to offer a coherent line of thought; each a sentence in itself, they can all be read apart from the short lines, which offer a more introspective meditation on poetry, sadness, and transcendence.
This particular line claims that humans produce the sweetest expression in song or poetry as a result of thinking on human sadness. Unlike us, the skylark sings a purer and more untarnished song of joy:
We look before and after,
And pine for what is not:
Our sincerest laughter
With some pain is fraught;
The line compares the nuance of human joy and sorrow with that of the natural world, which need not meditate on death, pain, or loss. The poem ends with an appeal for the bird to teach the poet how to "sing" as it does, so that he can teach the world the same lesson that he derives from the skylark's song.