Like many of Shakespeare's sonnets, this poem is an expression of love. In order to express your love, you have to talk about it, define it, examine it. In telling his mistress that he loves her, our speaker also has to give us an idea about what his love is like. This poem is partly about where love comes from, what motivates our feelings of affection for someone else. Specifically, it's about finding love in spite of (or maybe even because of) physical flaws.