A year ago last month, I welcomed former UK Prime Minister Theresa May to a community center tucked under a bridge in rapidly-changing south London. She was there to meet younger and older neighbors who were enjoying a quiz hosted by the charity I run, and to launch the world's first ever government-level loneliness strategy. As May mingled with octogenarians and millennials and shared local trivia over tea and biscuits, her strategy was being met with a mixture of derision, gratitude and curiosity. For critics, loneliness was nothing more than a natural and episodic personal emotion — an unavoidable part...