"How do I love thee?" is a poem about its own poetic nature, a list and catalog of all the different ways of loving that the speaker experiences. It's very important to this speaker to find phrases, metaphors, and language that can encapsulate her love, so that she can communicate its complexity to the beloved – and to the reader.
In "How do I love thee?", the constant, almost numbing repetition of the word "love" challenges the reader's familiarity with that term, making the word and the feeling strange and new.