PHILADELPHIA—Have you ever been to summer camp? My whole class went for a week when I was in seventh grade. I remember a feeling we all had towards the end, on the second-to-last day, of bleak and overwhelming melancholy. Something had happened. We didn’t know why, but what had begun with yearning and exhilaration on the first night in the cabins—the joyous conversation about girls and activities to come, unpacking all the underwear and towels nicely folded by Mom, ghost stories—ended with ennui and even dread. That week seemed to encompass years, even decades, of human experience.