Find the turn of Sonnet 73. What is its logical relationship to what comes before?

The closing couplet of sonnet 73 is like an admonition: one's love should grow stronger as one's time left to love is running out. It is not entirely clear whether this line is addressed specifically to the fair lord or in fact to himself, or perhaps even to both, since the narrator's approaching death will mean that each must bid the other farewell. In any case, the narrator is clearly distressed by his inevitable fate: old age, death, and eternal separation from the fair lord.