A speaker imagines how simple love could be in a "perfect world": you belong to me, I belong to you, and we're together. Then comes the blunt turn. We live in the real world, so none of that is true. The shift happens in a single plain line, "We're not," and the flatness of it feels like a door closing.
What follows is a short list of what the speaker misses: touch, warmth, and finally "us." That last item is the heart of the poem. The speaker isn't only missing a person but the pairing itself, the shared identity the two once had. It's also the only line without a comma or full stop at the end, which makes it feel like it just trails off.
The mood is quiet, sad and honest. There's no anger or blame here, just longing and acceptance. The simple wording makes the ache feel real, as if the speaker is saying it softly to themselves.