Like many of Shakespeare's Sonnets, Sonnet 73 is preoccupied with problems of aging and death, and nothing much uplifting (except the love part, we guess). Of course, we have no idea of knowing whether Shakespeare personally held these views, but this is definitely the outlook of the speaker of the poem. One thing we do know about Shakespeare himself, though: the guy was overflowing with ideas. Instead of letting potentially depressing topics like the inevitable end of human life get him down, he uses it as a stimulus to new and greater creativity.
You can see Shakespeare's creativity in the way each quatrain gives rise to an entirely new set of images: autumn turning into winter in quatrain 1, the sunset in quatrain 2, and a fire in quatrain 3. And there are even more metaphors contained within these big metaphors, as you'll see if you check out the "Symbols, Imagery, and Wordplay" section of this module.
This type of seemingly boundless inventiveness has Shakespeare written all over it. Finally, another major giveaway is the "Shakespearean sonnet" form, which we discuss in more detail in our section on "Form and Meter." What's important here is that the way Sonnet 73 divides its topics into 3 neat 4-line quatrains followed by a 2-line couplet makes it an absolutely classic example of Shakespeare's signature form.