In "Sonnet 20" the goal of the speaker is to praise the young man's beauty. Of course, throughout the Middle Ages and into the Renaissance poets have always praised the beauty of women. In a sense, beauty was always a special quality that women could possess but men could not. Shakespeare essentially agrees with that premise, that idea. And so, in this sonnet, he comments that the young man's beauty is so great that his beauty is like that belonging to a woman. He felt that there was no point in comparing the young man to other men since men do not possess beauty. Only by comparing the young man to the most beautiful of women can the speaker provide the reader with some clue as to just how beautiful the young man is.
The sonnet begins with the personification of Nature. Nature here is described as a creation goddess, responsible for creating not just plants and animals, but human beings as well. And, of course, Nature is responsible for the creation of the young man.
In the first line the speaker states that Nature gave a woman's face to the young man. The speaker uses the word painted to suggest that Nature is an artist and that the young man is her masterpiece. The speaker also calls the young man "the master mistress of my passion" (in line 2). The word master refers to a male; the word mistress refers to a female. The speaker means this as a compliment: the young man