The theme of Shelley's 1820 poem is the phenomenon of unrequited love.
In the first stanza, the speaker observes that in the natural world, specifically in the elements of water and air, there is no meaningful separation. He extrapolates this idea to a divine plan for everything in existence to have a counterpart.
In the second stanza, he employs further exemplification from nature, observing connections between earth and sky, waves in the sea, and flowers.
Each stanza ends with a rhetorical question that essentially asks the same thing: why won't you be with me?
The poem's tone is playful and imploring. The speaker tries to impress his beloved with lofty language and analogies that contain some amusing logical fallacies, but he does put a direct question to her--twice.