The many repetitions in the "Introduction" to Songs of Innocence mirror the poem's action and capture its gleeful spirit. Just as the child on the cloud demands to hear the same song over and over (taking a delight in repetition that anyone who has hung out with a small child will recognize), the poem's simple language spins giddily around the same few words.
From the very first lines, when the speaker comes "Piping down the valleys wild / Piping songs of pleasant glee," the poem returns and returns to the delights of having a pipe and being a piper. Listen to all the pipe-y polyptoton in the second stanza:
Pipe a song about a Lamb;
So I piped with merry chear,
Piper pipe that song again—
So I piped, he wept to hear.
The swift repetition of these words, with their clean alliterative /p/ sounds, summons up the sound of piping: a lively procession of little notes. It also suggests that piper and child alike are immersed in the joy of the music: the repetition of piping words matches the repetition of the piper's song.
Meanwhile, subtler, wider-spaced repetitions weave the whole poem together. In the second stanza, for instance, the speaker says that he "piped with merry chear"; in the third, the child seems of one mind with him when he requests that the piper sing his "songs of happy chear," as if he's hearing the words and catching the spirit of this very poem before it's even been written. That repetition underscores how closely linked speaker and child are, and how much their joy comes from creative interplay.
Another poem-wide repetition sprouts in the second stanza, when the child cries at the sound of the piper's "song about a Lamb." He "wept to hear," the speaker tells us—and then, when the speaker sang the same song, putting words to the melody, the child "wept with joy to hear." In the final stage of creation, when the speaker has written his songs down, he declares he's done so to share that experience with "every child," who may now "joy to hear" what the visionary child did.
Again, repetition suggests an experience transmitted and shared. First, the speaker and the child move each other; then, through a different kind of art, the speaker reaches out to "every child"—the reader very much included, whatever their age.