The speaker begins with a simple rule: if you don't love, you can't be hurt. So they teach themselves to feel nothing. Then they notice that everyone else seems empty too, as if people had all picked up the same trick without a teacher. The poem goes on to imagine why. It sketches the needs of early humans, food, fire and shelter, and adds isolation as a kind of protection from other people. From that, the speaker suggests, came fear and judgement, handed down without anyone meaning to teach them.
The voice that calls from the crowd, "come down from that mountain," shows how those who love are pulled back to safety, or brought down. The mood is cold, resigned and quietly bitter. In the final two stanzas, the poem turns personal. The big theory about humanity turns out to be rooted in rejection: being told "you couldn't stay" and "wasn't good enough." The emptiness is a wound dressed up as wisdom.