Sometimes at weddings the bride and groom write their own vows. Frequently, people say things like "my dear Amelia, you are my everything" or "my beloved Bill, I can't imagine myself going through life without you." Cheesy? Yes. But that doesn't make these lines any less true.
"To My Dear and Loving Husband" sounds like a short poem somebody would recite while standing at the altar about to get married. It starts out in a really formal and – we'll come right out and say it – boring way with three "if/then" clauses. Just imagine a bride reciting her vows in front of a whole bunch of people. She's probably a bit nervous, and isn't sure what to say at first, so she takes it slow.
After getting comfortable, our bride's poem starts to pick up steam. She finds her confidence and starts dishing all kinds of professions of love. She's starting to weep tears of joy at the altar as she exclaims in a powerful voice, "I prize thy love more than whole mines of gold […] my love is such that rivers cannot quench"! She uses the same powerful, emotional voice for the rest of her vows, finally concluding with this hopeful message: "Then while we live, in love let's so persevere / That when we live no more, we may live ever."
What we're really trying to say here is that this poem builds to one big, fat crescendo – the cymbal crash of the final two lines. But how does it build? It starts out slow and steady, by sticking to iambic pentameter strictly in the first six lines. Then, in lines 7 and 8, we get the odd, off-kilter rhyme pair of "quench" and "recompense." Our senses are sharpened, and now we're on the lookout for more tiny changes.
And tiny changes we get. First, there are the extra syllables in lines 10, 11, and 12. These extra syllables add just enough variation to play off the rhythm Bradstreet has already established. She's showing us a little style. Second, there's the introduction of alliteration in line 11, with the words "live," "love," and "let's." These sounds are slow and lingering. They give the lines weight, so that they can settle in our minds and have a real emotional impact. The fact that the rest of the poem has made little use of alliteration makes this single moment of it all the more powerful. Those "l" sounds jump right off the page and onto our tongues as we read those lines aloud.