In Sonnet 18, Shakespeare compares the beloved's beauty to a summer's day, much to the beloved's advantage. Although a summer's day may be very bright and beautiful, it won't LAST. Nor will the summer, for that matter. Like all the SEASONS, the summer will eventually FADE away and die, to be replaced by another season.

The same, HOWEVER, cannot be said of the beloved's beauty. His beauty is eternal; it will last forever. And that's because it has been immortalized in a lovely sonnet written by a great poet and playwright whose works will live on so long as people exist to read and enjoy them.

Sonnet 18 presents us with a contrast between mortal and immortal beauty. A summer's day, or indeed any form of earthly beauty, is a prime example of the former. It is mortal in that it will eventually die. The beloved's beauty, on the other hand, is immortal; it will never die.

That being the case, the speaker not unreasonably concludes that it would be inappropriate for him to compare the beloved's beauty with that of a summer's day. The two are simply not the same thing at all. They occupy entirely different realms of existence.