In Shakespeare's "Sonnet 18," why is the speaker reluctant to compare the beauty and youth of his patron with the summer's day?

In Shakespeare's Sonnet 18, more commonly known by its first line "Shall I compare thee to a summer's day?," the speaker is reluctant to compare his lover to a summer's day because he believes that she is "more lovely and more temperate" than summer days. The writer states that he prefers his lover to the summer season because she is not as violent as the early summer wind, or as intense as the heat. Additionally, the speaker argues that summertime is temporary, but the positive traits of his lover will last forever, stating:

But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st

Therefore, the poet is reluctant to compare his lover favorably to a summer's day because of the many ways she surpasses it.