The main theme of William Shakespeare's "Sonnet 73" is how a person is affected by growing older. In this sonnet, Shakespeare compares old age to the seasonal shift of autumn to winter, the passing of day to night and how a fire burns itself out when it is done burning.
The narrator in the sonnet also describes, to an unnamed lover, the role love plays as a person ages. The narrator tells his lover that aging and death should not destroy love as much as cause it to deepen. The narrator also describes how aging is harsh and empty, and by describing how aging feels, ultimately comes to terms with its inevitability.