dying fire of the persona's youth.

I read lines 9 and 10

(In me thou see'st the glowing of such fire
That on the ashes of his youth doth lie,)

as saying 'you see in me the glowing of a fire that lies on the ashes of *my* youth' (other readings of line 10 suggest that "his" relates to the person he addresses but this does not make sense to me for two reasons: first, he addresses the listener as "thou"; consequently, the corresponding pronoun should be "thy" (or "your"), not his; second, it makes no logical sense (to me) to given what follows).

It would make sense that the ashes of ones youth would be like a death bed that destroys (consumes) life even as it feeds it. The persona is old and is burning out, and the fire that burns him is what is left of his youthful passion.

My reading of the last two lines is that the person the poet addresses sees everything--the fire and the slow extinguishment of it, but still loves (in fact, loves with even more fervor because he OR she knows the love is marked by impending death):

This thou perceivest, which makes thy love more strong,
To love that well which thou must leave ere long

Line 12, "Consumed with that which it is nourished by" refers to life itself. We are all in the process of dying even as we live.

Line 13: "This thou perceiv'st, which makes thy love more strong." When one realizes the temporal nature of life, one can appreciate its fleeting state. (Perceiv'st = "to see").

Line 14: "To love that well which thou must leave ere long." Again, to love while one is alive is to really live. To understand life is not ours forever helps reinforce how precious each moment can be.

William Shakespeare's Sonnet 73 is a poem about love, like many of the sonnets. In this sonnet, Shakespeare is discussing the season, autumn, and how he has grown old.

In order to interpret the lines, one must understand the imagery to which he is speaking of. The lines in question are as follows:

Upon those boughs which shake against the cold,
Bare ruin'd choirs, where late the sweet birds sang.

The boughs which Shakespeare is speaking of is referring to tree limbs. Winter weather is moving in and the wind is shaking the branches. This could be looked at as personification given the branches are shaking against the cold (much like a person would do if under-dressed for the bitterness of the weather).  Therefore, line three is simply depicting branches moving about given the wind is forcing them to move.

Line four is still speaking about the branches. Here, the reader can see that the leaves have certainly fallen given the branches are described as being bare. The image Shakespeare sets up here is that the branches served as a choir stand which holds those who would sing. In regards to the birds, the birds are the choir who used to sit upon the branches and sing. Given the leaves have dropped and the weather has changed, the birds no longer perch on the branches to sing. (This is denoted by the use of the word late, meaning no longer.)