Alliteration fills the poem with bold, bright music that evokes the sensory splendor of spring—and gentler sounds that suggest the speaker's sorrow.
Listen to the breathy /h/ alliteration in the first stanza, for instance:
I dreaded that first Robin, so,
But He is mastered, now,
I'm some accustomed to Him grown,
He hurts a little, though—
These whispery, soft sounds make it sound as if the speaker is exhausted by her grief.
By contrast, spring is mighty. Listen to the plosive /p/ sounds in lines 7 and 8:
I thought if I could only live
Till that first Shout got by—
Not all Pianos in the Woods
Had power to mangle me—
That /p/ sound seems to pop out of the stanza, stressing the spring's sheer force. Similar powerful, blunt alliteration appears when the speaker says she "dared not meet the Daffodils" and "could not bear the Bees should come": whenever a new springy beauty appears, it's announced with bold, overwhelming sounds.
The poem's alliteration thus becomes a playful, insistent presence—suiting the speaker's impression of spring as a mercilessly noisy, powerful parade of cheery living things.