There is a parallel drawn between …

The poem draws a parallel between rain and music. The line, "I am the Poem of Earth", said by rain, reflects a connection between rain and poetry. Just like a poem brings joy to its poet, similarly, rain brings joy to its origin, the earth. This connection becomes more conspicuous in the final two lines. "(For song, issuing from its birth-place, after fulfillment, wandering Reck'd or unreck'd, duly with love returns)".
In these lines, the poet draws similarities between rain and music observing that the life-cycle of rain and song are alike. Both are perpetual in nature. Moreover, the sound of the soft-falling rain is in itself a kind of music.
The song issues from the heart of the poet and travels to reach others. It wanders and, whether heard and enjoyed or not, eventually returns to its creator with all due love. Similarly, rain originates from the earth, and after fulfilling its tasks, returns to its own origin only to spread beauty, purity and love.