What lessonn does William Shakespeare offer to the addressee of “Sonnet 73”?

Shakespeare's Sonnet 73 suggests that through aging and death, we can and should appreciate what we have. The basic idea is that because we will lose our lives and our loved ones, and the seasons will change, we should try to love what we have while we still have it. 
In the first quatrain, the speaker says,

That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruin'd choirs, where late the sweet birds sang.

He is comparing his own life to the changing seasons in the first line: "That time of year thou mayst in me behold." This is the time when, like autumn, the leaves change colors and drop from the trees. The birds have left. Where there was once life, there is now a lack.
 
The second quatrain continues,


In me thou see'st the twilight of such day
As after sunset fadeth in the west,
Which by and by black night doth take away,
Death's second self, that seals up all in rest.



Here, the speaker compares himself to the end of a day, instead of the end of a season. He is like the "twilight" when the day is dying out and night is coming to take over. He refers to the night as "Death's second self," making the comparison even more explicit.
 
The third quatrain reads as follows:
 

In me thou see'st the glowing of such fire
That on the ashes of his youth doth lie,
As the death-bed whereon it must expire,
Consum'd with that which it was nourish'd by.



Here, he addresses another person directly with the use of "thou." The speaker is saying that "you" can see a similar dying process in the speaker himself, described through the metaphor of a waning fire. The very body that gave him life will soon be no more.
 
The poem ends the the couplet: 
 

This thou perceiv'st, which makes thy love more strong,
To love that well which thou must leave ere long.

Because you understand that the speaker is dying, you can increase your love. The fact that the speaker will not be alive much longer makes the you the speaker addressed. The implicit lesson is that we should appreciate what we have because one day all things and people we love will be gone.