The reason that this particular love poem is so popular is because the images and ideas expressed are universal — being head over heels in love! In the poem, the beloved becomes elevated to the level of god-like adoration. Nature's beauty, ("buds of May"), can be altered by the elements, and even the sun in the sky can be 'dimm'd,' but the beauty of the beloved is more powerful and can never fade.
Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd;
And every fair from fair sometime declines,
By chance, or nature's changing course untrimm'd;
But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st,
Nor shall death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st;
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.