I know its death, but age is my answer, and I know its not the popular answer, and I know he talks about death, but I believe age makes a love stronger.
He compares himself to a tree in the Autumn: “Upon those boughs which shake against the cold.” He explains that the sun, symbolizing life, in him is fading and the night, symbolizing death, is taking over. These metaphors depict the inevitable aging process of the mortal human being. However, he knows his lover still sees a fire in him but suggests that it will go out soon as the fire is consumed. The speaker knows his lover sees him getting older, but believes it makes their love stronger as they cherish their last moments together before the speaker dies.
The inevitability of death.
In this beautiful sonnet, Shakespeare speaks of youth gone by as a fire that once glowed bright but now is nothing more than ashes; ashes that form a deathbed for the speaker where it will all expire consumed by what once was but, inevitably, had to perish and pass on, like everything and everyone. The inevitability of death does make this terrible sight for the one that loves, beautiful and more powerful nonetheless.