Answer:
The inevitability of death.
Step-by-step explanation:
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.