Why did Shakespeare focus on the ways his subject was different from summer instead of describing how they are similar? Sonnet 18

He chose summer because there's a downside to summer, but the woman has no downside.

Explanation:

The poem starts out by asking "If I compared you to a day in summertime, would that be a good way of expressing you beautiful you are?" Then it answers its own question by saying, "No, because you're even more beautiful than that." Then it talks about various ways in which summer's beauty is imperfect and doesn't last. Then it says, "Your beauty, on the other hand, will last forever, because by praising you in this poem, I am making you immortal."