What are the poem's principal parts? Why does the Petrarchan model suit this sonnet?
Edna St. Vincent Millay in "What Lips My Lips Have Kissed," reverses the usual poetic roles of men and women, and features a female speaker reminiscing about past lovers, rather than the opposite.
In the first part of the poem, the octave (first eight lines), the speaker has forgotten her past loves, but is reminded by the rain (metaphorically, ghosts) knocking on the window pane and listening for a reply. For those she has forgotten, she feels a quiet pain.
The beginning of the sestet is signaled by the "Thus" of line nine. In the second part of the poem two seasons--winter and summer--are used to symbolize her state. In the poem's present the speaker is like a tree in winter that doesn't remember the birds that used to perch on it in the summer. But there was a time, for a little while, like the tree in summer filled with singing birds, when summer sang in her and, metaphorically, lovers visited her.
The Italian sonnet form is used to divide the thoughts in the poem into two parts. The first part starts with the general "I have forgotten" and moves to the specifics of one night. The second part takes the specific and applies it to the general.
Thus, the sestet applies the thoughts of one night to her state of existence concerning past loves, and the lack of present loves.