"Infant Joy" uses intense repetitions to create an atmosphere of harmony and delight. Returning and returning to the same phrasings, the poem reflects the two speakers' endless wonder at the "joy" of new life.
Some of the most pronounced repetitions in the poem come in the form of parallelism and anaphora. For instance, almost every line the baby speaks begins with an "I":
I have no name
I am but two days old.—
[...]
I happy am
This anaphora reminds readers that, at only "two days old," this baby is still getting used to being an "I." The baby is an independent spirit, sensing the miracle of its own being.
The adult speaker, meanwhile, can hardly contain their delight at the baby's joy. Parallelism, anaphora, and full-on repeated lines make them sound as if they can't stop marveling over just how wonderful the baby is:
Sweet joy befall thee!
Pretty joy!
Sweet joy but two days old,
Sweet joy I call thee;
[...]
Sweet joy befall thee.
Notice that the adult speaker doesn't just echo their own earlier words, but the baby's, calling back to the idea that the baby is "but two days old." In fact, readers who put these stanzas side by side will notice that their many repetitions create a kind of symmetry. The adult speaker closely echoes the baby, suggesting the two share a loving, empathetic bond.
Last but far from least, the word "joy" appears six times in this short poem. This diacope is downright ecstatic: the poem is quite literally full of "joy."