The poem is about love, not between the writer ( Shakespeare) and a lover but rather as a concept and an idea. He wants to show that love is constant in life and does not change over time.
There is a recurring theme of nature in Shakespeare's sonnets.
He uses the metaphor of "the sky" to show the relation to love. Like the stars love can be observed across the globe throughout time. It is not restricted by time place and can exist despite these.
He talks about love and time as conflicting enemies. I think that he uses this really well throughout the poem.
Sonnet 116 is very close to some of the other sonnets written by Shakespeare such as To His Coy Mistress. Where they both explore the theme of love and time.