In the first two quatrains of Sonnet 18, the speaker wonders if he should compare his love to a summer's day. But, he reflects that the beauty of Nature is too temporal: "Rough winds do shake the darling buds of May,/And summer's lease hath all too short a date." In addition, sometimes the heat of summer is oppressive or its beauty "decline."
Then, in the third quatrain, the speaker declares that his lover's "eternal summer," or soul, "shall not fade/Nor lose possession of that fair thou owest/Nor shall Death brag thou wanderest in his shade." Even when his love has died, the speaker avows, the beauty of his love will endure. The final couplet explains, "So long as men can breathe, or eyes can see,/So long lives this[my poem], and this gives life to thee.
A truly beautiful declaration of eternal love!
Sonnet 19 reiterates this same declaration that the speaker's love will be eternally young and beautiful because "My love shall in my verse ever live young" no matter how much Time carves "thy hours my love's fair brow."