Sonnet 90 has the rhyme scheme we identify as typical of the Petrarchan sonnet: ABBAABBA CDEDCE. Often commentators will regard the first eight lines, the octet, as expressing the basic premise of the poem, while its concluding six lines, the sestet, provide a kind of answer, a response to the ideas put forth in the octet. In this particular case the octet does ask a question:
Why marvel that I so suddenly burned up?
The speaker is saying his beloved is so beautiful that it's no wonder his love for her burned so quickly. The concluding sestet does not exactly answer this question, but seems to amplify even further the hypnotic attraction the beloved presents to him, then concludes with an acknowledgment that the wound from his love has not healed, though the beloved is longer the "celestial spirit" she once was.
The sestet thus presents a change of sorts from the octet, given that the beloved herself has changed in some sense, though the...
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