After reading several of Pablo Neruda's poems and studying his personal life for my presentation, I truly appreciate the beauty of this poem. I don't know when this was written, but I do know that Neruda had several wives and mistresses. I don't think this happened because he was a bad person and couldn't keep a committment (although the committment argument can't be thrown out completely, only temporarily for the sake of my argument :) I think he was one of the few who could find beauty in everything and everyone. The way he describes the stars and the sky is as passionate as when he describes the "nights like this one I held in her arms".
I also find this poem interesting because of how he describes his love for his former lover, and specifically how this love changed over time. In line 9, he says, "...sometimes I loved her too." If you only "sometimes" love someone, it is strange that he is writing a poem with the "saddest lines" about someone he has lost. In line 23, he says, "I no longer love her, that's certain, but how I loved her." Again, it is strange that he is writing this poem to her despite the "certain" (imagine me making quotation marks with my hands) fact that he no longer loves her. If this poem was written to a former mistress, and his wife were to read this, I think Neruda would be busted!