"The Saddest Poem" by Pablo Neruda is about lost love. The speaker laments over the loss of a lover, not who has died, but who has moved on from him. His strong connection of the poem to the stars and sky suggests the insurmountable distance between him and his former love:
I can write the saddest poem of all tonight.
Write, for instance: "The night is full of stars,
and the stars, blue, shiver in the distance."
Because he is no longer with his love, he is able to write the "saddest poem of all."
At the end of the poem, the speaker offers some hope for himself. While the body of the poem is very sad and depressing, he is purging himself of her and has decided that this is the last time that he will think of her. This is the last night that he will lament,
Because on nights like this I held her in my arms,
my soul is lost without her.
Although this may be the last pain she causes me,
and this may be the last poem I write for her.
In the morning, we get the sense that he will move on and begin his life anew. His "saddest poem" is the final purge of a wonderful and difficult time in his life.