Go through the first couple of quatrains once again. The niche are put so we are informed that he otherwise she is much more stunning than simply an excellent summer’s time. The fresh defects of summer’s date is actually outlined.
Look at the 3rd quatrain. They starts with the word ‘but.’ One scratches a change out of emphasis. Now new subject’s eternal charm was emphasised.
Look at the couplet. It’s a summing-up – an assurance the subject’s charm can last so long and there’s people on the planet. A good rhyming couplet during the English poetry is often extremely effective, plus an excellent sonnet, it couplet figures up and rounds from the poem. It can be used to get increased exposure of part of the suggestion, or to undermine it, or even to render a funny position.