The sonnet structure is particularly important in this poem.  A sonnet is generally a fourteen line poem, in this case split into two sections: an octave (eight lines) and a sestet (six lines).  That structure, derived from an Italian origin (hence the naming of this type of sonnet as Italian or Petrarchan) establishes an implicit logic to the poem.  The opening octave represents an argument or thesis, the closing sestet resolves the argument or reveals the subtleties of the thesis.  The shift from the octave to the sestet is called the volta.