The first two lines of "My Heart Leaps Up" establish the event that prompts the rest of the poem: the speaker relates how his heart "leaps up" when he sees a rainbow. Rather than simply say outright that seeing a rainbow makes the speaker feel happy or joyful, he uses personification to describe his reaction. Hearts, while one of the most essential parts of the human body, do not themselves "leap." By animating his heart, however, the speaker is able to portray the sudden bliss the sight of a rainbow causes him to feel. It also introduces a sense of playfulness and innocence—qualities often associated with childhood—from the very start of the poem.
The heart's leaping also suggests that the speaker's joyful reaction is not planned, but rather is spontaneous, unbridled, and perhaps even inevitable. That is, the speaker cannot help but be happy upon seeing a rainbow, because his heart seems to respond of its own accord.
In turn, the inevitability of the speaker's reaction to the rainbow is reinforced by the musicality of the poem's language. These opening lines employ assonance and consonance to links the words together. Note how the many long /ee/ sounds, when spoken aloud, stretch the mouth as in a smile, while the plosive /b/ and /p/ sounds add a bouncy rhythm to these lines:
My heart leaps up when I behold
A rainbow in the sky:
This repetition of similar sounds gives the poem the impression that the words inevitably belong together, reinforcing the naturalness, or honesty, of the poet's response to the rainbow. The poem also employs enjambment between these first two lines. The reader easily glides past the first line break into the second line to find out what, in particular, makes the poet's heart leap so.
Additionally, the first line establishes the meter of the poem as iambic tetrameter. This just means that each line contains four iambs, poetic feet with an unstressed-stressed, da DUM, beat pattern:
My heart leaps up when I behold
The second foot of the first line might also be scanned as a spondee, or two stressed beats in a row—"leaps up," which emphasizes the joyous bounce of this movement. The meter is broken slightly in the second line, however, as it only has a set of three stressed and unstressed syllables (making it a line of iambic trimeter):
A rainbow in the sky:
This break in the meter evokes the poet's heart skipping a beat.