Sonnet 18:

The author writes about a love that cannot be compared to anything, and that will live on forever. In this sonnet love is compared with high, unrealistic, and unattainable beauty. The primary enemy of beauty is time, and although it might fade, it can still live on through a person's memory or words of a poem. The sonnet is immortalized with the speaker’s promise that his love will live on and that it will remain perfect not in the eyes of the beholder but the eyes of those who read the poem.

“And every fair from fair sometime declines,
By chance or nature’s changing course
But thy eternal summer shall not fade…
When in eternal lines to time thou growest” (Gardner, et. all, 465)

Sonnet 73:

In this sonnet, the author is leading the reader line through line to his death. He uses a direct comparison between the seasons and relates it to the different stages of life. The poem contains images to contrast his ageing, the life in him is fading and death is taking over as he runs out of time. This sonnet is the story of the speaker slowly coming to grips with the finality of his age and his impermanence in time.
“In me thou seest the glowing of such fire
That on the ashes of his youth doth lie,
As the deathbed whereon it must expire,
Consumed with that which it was nourished by” (Gardner, et.al, 466)
Sonnet 130:

In this sonnet what is important to the author is the natural beauty of his mistress.   This poem is an expression of love, he is describing his mistress the way she is, not stereotyping her in an unrealistic way. He makes clear that her appearance isn't crucial, but most of his feelings about her are not reveal. The author understands that his mistress is not perfect or even special in a certain way; instead he sees who she really is and loves her for that.
“I love to hear her speak, yet well I know
That music hath a far more pleasing sound” (Gardner et.al, 467)