In Sonnet 18, the speaker describes how the qualities of his beloved are better than the qualities of nature ("a summer's day"). First, he notes that 'she' (his beloved) is "more lovely and more temperate" than a summer's day. Summer is too short and the violent winds rattle the late buds of spring. The significant element here is the notion of change; summer can be too hot or cloudy. In the sublunary (earthly) world, things change; nothing is permanent. "Every fair from fair sometime declines," (every lovely thing in the loveliness of nature fades).
The speaker has written this sonnet to express his love; it is the expressing that is potentially eternal. Unlike the temporary existence of nature where things fade over time, the speaker notes that his beloved's beauty or existence will never fade:
But thy eternal summer shall not fade
Nor lose possession of that fair thou ow'st,
In writing the sonnet, the speaker is attempting to immortalize his beloved. The poem is therefore a tribute to her, a potentially eternal declaration of his love and her being. As long as men continue to read, in the future, they will give life to the "eternal lines" the speaker has written. His beloved's "eternal summer" remains eternal, living in the poem itself:
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.