I believe that the theme of the poem "To a Skylark" by Percy Bysshe Shelly is that humans cannot possibly feel the carefree joy the skylark feels each and every day as we are incapable of staying in the moment.
The poem begins by calling out to the skylark, calling it a "blithe" spirit. To be carefree, one can have no worries, no concerns. The speaker in the poem praises the song the skylark sings calling it "unpremeditated art". Spontaneous song clearly emphasizes the bird's ecstatic joy.
The speaker follows the skylark's journey: it "springs" from the earth, then flies into the "deep blue" or sky, and as it does this, the bird continues to sing. This movement again emphasizes the unbridled happiness the skylark embodies. The speaker continues to track the bird: "In the golden light'ning/Of the sunken sun". This show how quickly the bird is flying and the next line ""Thou dost float and run" again shows how filled with joy this bird appears. The bird then continues to fly, and the into the sunrise or sunset (I'm not sure which, but I think it is the sunrise) as the "pale purple even/Melts around" the bird, and while the bird can no longer be seen, the song can still be heard. This shows once again how the happiness just bursts forth and can't be contained in this little skylark.
The speaker later points out that all the joy the bird experiences is unattainable for human beings - "What thou art we know not" - and then compares the skylark to things who demonstrate some measure of happiness, but still not equal to the joy the bird possesses: the raindrops from clouds that later produce rainbows. The bird is like a poet no one pays attention to until people appreciate it after disregarding the real hopes and dreams of the world. The bird is like a princess in a palace who is thinking of her love in her private quarters and like a glowworm secretly spreading its light and joy, and so forth. These comparison all "shine" with happiness whether anything or anybody else takes notice. They are "in the moment"
The speaker then ponders what causes this bird to be as happy as it is stating " What objects are the fountains/Of thy happy strain?" The speaker goes on to say what the bird has never experienced: laziness, annoyed, heartbreak, even death must not hold the fearsthat it seems to for humans. The speaker says "We [humans]look before and after" which probably means we are never just in the moment. We always want what we don't have - "pine for what is not". Even when we laugh, it is often filled with pain. Our best songs talk about sad things
The speaker then goes on to say that even if we could eliminate scorn, pride, and fear - all the things that often cause humans sadness, we still would never be able to be as happy as the skylark appears to be, and if this bird could teach humans to be half as happy, it is certain the world would listen to him just as he is listening to the skylark.