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 speaker is still enraptured by her. We therefore see a typical polarity between octet and sestet, with the latter altering the idea presented in the former, but not really presenting an opposition to it. Each section is, of course, unified by its rhyme scheme. Unlike in the later form used by the English poets, the Elizabethan sonnet, the octet is unified by the use of only two rhymes. Since the Italian language as a whole contains more rhyming words than English does, it's more natural for an Italian poet to use the ABBAABBA rhyme scheme for the octet, and this facilitates the cohesiveness of those lines and the unity of the opening thought they express.

Petrarchan sonnets, also called Italian sonnets, were made popular by Petrarch, a poet of the 1400s, who wrote poems of unrequited love. The structure of his sonnets consists of fourteen lines: first an octave (which is eight lines) with a specific rhyme scheme abbaabba, followed by a sestet (six lines) with a more flexible scheme.

In the octet, the woman, Laura, is described in intense, exaggerated detail as "the sweet light beyond all radiance," thus introducing the theme of the sonnet. It becomes apparent that no physical love exists as the line "love's tinder in my breast unburned" indicates. The problem is thus introduced to the reader.

The sestet usually resolves the problem whereas in Petrach's Sonnet 90 it reveals the permanence of his problem. His love will never be realized as "the wound's not healed," revealing his very real pain despite her transience - "she bore an angel's form."

Petrarch indicates that she is no longer in his midst and his tone is wistful - she has an almost 'heavenly spirit" -even though thoughts of her almost make him ill.  

There are various translations of the sonnet but Petrarch's influence is undisputed. Shakespeare recognized his art form when composing his own earlier poetry and even his plays.