Repetition draws attention to a number of important words in "Lullaby": "love"/"lovers," "human," "beauty"/"beautiful," "mortal," and "vision." These repeated words capture the poem's balance between romance (its celebration of love, beauty, and the visionary feelings they inspire) and realism (its recognition that all people, including our loved ones, are flawed and mortal). There are also several references to "night"/"midnight" and "day," which underscore the poem's setting: the hours between nightfall and dawn, when lullabies are typically sung.
In fact, the words "beauty," "vision," and "midnight," which appear respectively in the first, second, and third stanzas, reappear in the first line of the last stanza: "Beauty, midnight, vision dies." This repetition has the effect of braiding together themes from the earlier stanzas into a unified, conclusive whole. All these things pass, the speaker notes, but they can still nourish us at the dawn of a new "day."
Repetition also adds emphasis to certain phrases. Look at the anaphora in lines 29-30, for example:
Not a whisper, not a thought,
Not a kiss nor look be lost.
Here, the repetition conveys the speaker's determination—or desperation—not to lose a single memory from this passionate evening.
The poem makes effective use of parallelism as well. Notice how the parallel structure in lines 37 and 39 helps the speaker draw both a comparison and a contrast:
Noons of dryness find you fed [...]
Nights of insult let you pass [...]
That is, the similar phrasing highlights both the contrast between "Noons" and "Nights" and the emotional parallels between creative "dryness" and painful "insult." Basically, the speaker is hoping that their shared passion—and the joys and memories it brings—will help comfort the lover through bad days and bad nights alike.