The biggest change is that for the first time in history it's easier to live alone than in a family. 100 years ago good luck trying to grow anything on a farm without any help, and most children were free labor that would later convert into free retirement care. Nowadays you don't need others for basic survival, and children are a huge liability. With a little bit of creativity you can live in a city yet go a month without talking to anyone, and still have your biological needs taken care of. So many people choose not to, because truth is, most people don't like each other, it's just that in the past they'd either somehow get along together, or die. Nowadays they don't have to.