The themes of both sonnets XVIII and CXVI are the same: The immortal quality of true love. For instance, in Sonnet XVIII, the speaker declares,
But thy eternal summer shall not fade,/Nor lose possession of that fair thou owest;/Nor shall Death brag thou wander'st in his shade,/When in eternal lines to time thou growest;/So long as men can breathe, or eyes can see,/So long lives this, and this gives life to thee.
Likewise, in Sonnet CXVI, the speaker affirms this immortality of love in the final quatrain and ending couplet:
Love's not Time's fool, though rosy lips and cheeks/Within his bending sickle's compass come;/Love alters not with his brief hours and weeks,/But bears it out even to the edge of doom./If this be error, and upon me prov'ed,/I never write, nor no man ever lov'd
Notwithstanding these declarations of the immortality of Love, it is apparent that the assertions of the speaker in both sonnets are emotional equations rather than logical deductions. In Sonnet XVIII, for instance, there is no way of proving the conclusion made in the last quatrain that poetry has the transformative power of giving life to its subject. And, in Sonnet CXVI the first lines establish a conditional state, requiring the reader to cast aside the possibility of "impediments to the marriage of true minds," obstacles in the path of the union of two people. But, if the reader rejects this condition, then the conclusion of the poet that love does not change over time is not necessarily self-evident as the poet asserts.
Several comparisons can be drawn between both sonnets. There is comparative language employed that seek to link love to natural phenomenon in both sonnets. In sonnet 18, love is compared to the notion of a "summer's day" while in sonnet 116, the parallel is to "the star to every wandering bark." The idea of evoking the natural world in articulating the condition of love is a similarity that is evident in both sonnets. Another similarity in both sonnets lies in their closing couplets. There is a tone of commitment and certainty in the respective sonnets' assertion about the articulated nature. In sonnet 18, the love that has been suggested is as valid as "men can breathe and eyes can see." This asserts a position of understanding and commitment that is difficult to deny. This same element is seen in sonnet 116, where its couplet argues that the vision of love that has been so accurately depicted that if it is incorrect, "I never writ and no man ever loved."