In the first stanza of this deeply felt or lyrical poem, the speaker describes how dearly he loves his beloved, so much so that he experiences her in everything he sees. Yet, he says to her in stanza 3, if you stop loving me, I will stop loving you. Then he states:
you forget me
do not look for me,
for I shall already have forgotten you.
Finally, in the last stanza, he says,
my love feeds on your love, beloved.
Neruda needs his lover to return his love because he understands and tries to convey to her that love is reciprocal. He likens love to a "fire" that has to be fed, or it will inevitably go out. What feeds love are the gestures that show love: he needs her kisses, and he needs her arms around him.
The theme of the poem is the same as the theme of the line in a song by The Beatles which says, "The love you take is equal to the love you make." Love, he understands, no matter how deeply felt, cannot last long without return. He is expressing a sentiment opposite to those poets who thrive on unrequited love, on a longing for love that is not returned. For Neruda's speaker, such a relationship will not work.
Neruda thus expresses not only the beautiful side of love but also the practical fact that love takes work on both sides if it is to stay alive.