One of the first things people like to point out about "How do I love thee?" is that the words "I love thee" appear in eight of the poem's fourteen lines – more than half. The word "love" occurs a whopping ten times. So, the first thing we notice about the sound of this poem is that it's repetitive; instead of using synonyms for love, other words to describe love itself, the poet is interested in characterizing different ways of loving.
What do we mean by that? Well, instead of distinguishing between love, passion, affection, and so on, this speaker wants to distinguish between loving "freely" and loving "purely" and loving in various other ways. The sound of the poem reminds us that the distinctions being made here are of degree and method, not of the thing itself. That is, we're not distinguishing between different things – the only thing being discussed here is, well, love. What varies is the way the love works and how intense it actually is. The thing itself – the word "love" – always remains consistent, reminding us that the speaker's love for her beloved is a constant and unchanging thing. It varies, of course, but it doesn't actually change. The phrase "I love thee," repeated until it almost becomes unfamiliar, is the backbone of this sonnet even more than the rhyme scheme or meter.