Reflection for Sonnet 73

This sonnet's theme is aging. The second line references autumn, whose parallel is the decline in life after one's prime. Ruined choirs could mean how the choirs deteriorated quickly after Henry VIII died, which is another way to talk about life's ending. In the first line of the second quatrain, the speaker talks about himself. He says that he is in the twilight of day, which means that he is getting old. Then he talks about the sunset fading and that the black night takes away. I think that he is saying that he's going to die soon. Death's second self could mean sleep, which is seen as a living death.
The third quatrain seems to be one metaphor. Inside the speaker there is a fire, and that fire is dying. The fire must die soon; its downfall is that it is being nourished by what it needs to live.
The couplet is telling the person to which the poem is directed to that he needs to appreciate his youth before it fades like the speaker's has. The last line could be telling the young man that he needs to love that which will soon die, which could mean the speaker.