The central theme of Shakespeare's Sonnet 73 is that of agape. The friendship sonnets trace true friendships based on agape. In Sonnet 73, the poet is preparing his friend for the death of his youth and passion, metaphorically speaking. In the first quatrain, the poet compares himself to autumn. These references establish what the poet thinks the young man sees as the young man looks at him. Like the changing colors and the falling of leaves in autumn, the young man notices that the poet is falling from his youth and passion. Perhaps even the color of the poet's hair is changing from the original color to gray and possibly falling out because old age is setting in. Like the "Bare ruin'd choirs, where late the sweet bird sang," the poet once was 'sweet' and full youth and passion, but now he is the empty shelter where the youth and passion once was stored.
             In the second quatrain, the poet compares himself to the end of a day, or twilight. This reference also establishes what the poet thinks the young man sees as the young man looks at him: the fading youth and passion of the poet as if it were the faint afterglow of the fading sun. In the line "Which by and by black night doth take away," the 'black night' is a metaphor for death itself. As 'black night' is taking over the remaining light of the day, death is taking the remaining youth and passion that is left in the poet. "Death's second self," which is thought of as sleep, "seals up all in rest," or puts everyone at rest.
             In the third quatrain, "In me, thou see'st the glowing of such fire" means that the young man still sees the youth and passion that the poet has left in him. "Than on the ashes of his youth doth lie" refers to the glowing embers that are left to burn out at the end of a fire. In the line "As the death-bed whereon it must expire," the it refers to the poet himself. In the line "Consumed with that which it was nourish'd by," the that refers to the poet's desire...