Sonnet 116 claims that true love is not based on practicality, pragmatism, or any other material standard, but rather that true love exists beyond these concerns, beyond change ("alterations") and beyond time ("Love's not Time's fool"). Rather, true love is completely and eternally fixed ("an ever-fixed mark"), unmoving, and unaware of the material, physical world around it.
In lines 7 and 8, Shakespeare uses the metaphor of the star to make the important argument that while we can look at true love like we can observe a star in the sky ("his height be taken"), we cannot assign it a value (its "worth's unknown"). One of the points of this sonnet is to say that while humans have an innate sense that true love exists, our attempts to describe true love and explain it always come up short. Unlike true love, our lives are flexible, moveable, physical, and temporary, which means that we will never be able to describe the actual worth or experience of true love in words. For this reason, the poem does not, for the most part, describe what true love is; it describes what true love is not. The negative sentence structures begin in line 1 when the speaker says, "Let me not" admit that true love has faults. He proceeds to describe true love as not alterable, not bending, never shaken, and not "Time's fool." The one positive sentence in the poem—and, once again, the poem's central argument—occurs in lines 7 and 8:
It is the star to every wand'ring bark, / Whose worth's unknown, although his height be taken.
True love can only be described through a metaphor of something else that is beyond our human conceptions of space and time, a star. True love is not subject to the powerful forces of time, but we humans, with our "rosy lips and cheeks" that eventually fade, are affected by time. Therefore, it is difficult for us to explain true love, although we know it and can feel it, just as it is difficult for us to explain the nature of a star, though we know what a star is and can see it above us at night.