My mistress' eyes are nothing like the sun
Mistress is a term used during Shakespeare's era to address a wife or a sweetheart. In the poem, the speaker is describing his mistress or wife. He is comparing the features of his mistress to what is beautiful in his eyes, like the sun, coral, snow, rose, among others. In his description, his mistress is just the opposite of what he perceived to be beautiful.
The speaker describes his mistress as pale, having lips that are not red like the corals, eyes that are not bright, cheeks that are not red like roses. The speaker was even a bit insulting his mistress for having a breasts that are not white as snow, hairs that are not shiny and straight as wire and when she walks her feet tramples on the ground. Moreover, when her mistress speaks her breath smells odorous and her voice is not as soothing as music. Simply put, her mistress is physically ugly.
However, despite all the qualities of her mistress, the speaker loved her above all. The speaker shows that his love is rare and true for despite the physical appearance of her mistress his heart beats for his unconditional love towards the lady. Unconditional love means loving a woman for what she is, for who she was and accepting her imperfections because afterall, love takes all these imperfections away.
In saying that his love is “rare”, he meant that he finds fulfillment in the love of his mistress. The speaker personifies a simple husband who doesn't look into the physical attributes of his wife but listens to what his heart dictates. This husband is one of a kind because he depicts somone who looks into the personality and depth of his wife, rather than physical appearances. Thus, his love for his wife makes her the most beautiful woman in his life.
Shakespeare, William. “'My Mistress' Eyes are Nothing Like the Sun.”