“There was no singles problem until singles got so single-minded that they stopped wasting time with anyone ineligible. Before that, it was understood that one of society's main tasks was matchmaking. People with lifelong friendships and ties to local nonprofessional organizations did not have to fear that isolation would accompany retirement, old age, or losing a spouse. Overburdened householders could count on the assistance not only of their own extended families, but of the American tradition of neighborliness.”