Answer :
The Sonnet use rhyme scheme of abab, cdcd, efef,gg. It is believed Shakespeare addressed this sonnet to a younger lover. I believe the theme is We love something we know is going to be gone soon more than we love it in it's peak. It's the perceptive of someone that love something that he's sure gonna be eventually gone/ taken away form him.
Shakespeare mainly uses metaphors and we can observe it at the end of each stanza. In the first stanza of the sonnet Shakespeare compares his aging to autumn with yellow leaves, loosely hanging from the tree. Then, in the next stanza he equates the night to death with the comparison of his life to twilight of the day. In the third stanza, Shakespeare shows how his energy is dying out in a death-bed.
In the final couplet of the sonnet, Shakespeare writes that:
This thou perceivest, which makes thy love more strong,
To love that well which thou must leave ere long.
Shakespeare shows that his lover will continue loving him although he is growing older and fading away slowly.