Who wrote, "Our sweetest songs are those that tell of saddest thoughts"?

"We look before and after,

And pine for what is not:

Ours sincere laughter

With some pain fought;

Our sweetest songs are those that

tell of saddest thought."

This stanza is taken from P. B. Shelley's poem "To a Skylark", and is deeply philosophical and reflects on the grave truth of human life. This is because man can never be truly happy. His Joy of the present is shadowed by the memory, sad or sweet, of the past, and the fear of uncertainty about the future. Man seems to be haunted with a sense of restlessness and frustration and enjoy what he earnestly desires. Naturally, his laughter, even when this is thoroughly sincere, is not absolutely free, unmixed, but has a bitter tinge of sadness in it. Indeed, the quintessence of human life is found to lie in the profound sense of sadness and that is why all the great literary and artistic achievements of man - his great poems, plays, paintings, and songs are on the great tragedies of human life. Sadness, and not joy, echoes best the inner urge of the human heart and prevents man from singing as spontaneously, as joyously, like the skylark, that has a clear vision of life and death.

"To a skylark" expresses a paradoxical statement which echoes the poet's despondent mood. Human existence is very paradoxical in that it presents both the vitality of joy and despondency of sadness at one go and hence it is the endeavor of each and every artist to capture such meditative pleasures, troubled joy's.