Millay uses metaphor once in "What lips my lips have kissed, and where, and why" to intensify the speaker's state of mind by reflecting the speaker's mood upon the world around her.

In lines 3-5, the speaker listens to the rain outside her window. She compares the sound of the raindrops falling against on her window to the sound of ghosts "tap[ping]" and "sigh[ing]" on the glass. Ghosts are associated with feelings of loss, pain, nostalgia for the past, and sorrow. As the speaker is currently mourning the loss of her memories of love, she is consequently feeling loss, pain, nostalgia for the past, and sorrow. It is no wonder, then, that the sound of the raindrops seems to the speaker more like the sound of ghosts instead.

Moreover, in this metaphor, the ghosts demand the speaker's attention ("tap and sigh") and wait for her "reply." The ghosts are symbolic of what the speaker has lost—namely her past lovers and memories of love. Yet though they demand her attention, the speaker is unable to remember her past lovers or her memories of love. Thus, these ghosts do not comfort the speaker with their presence. They serve only as reminders of what she has lost and can never regain.