1. Colleges get expensive and matter a lot, so people leave home for college if they want to achieve. Time to make new friends.
2. After college, a lot of people go where the job is-- this might be far from your family and since you went to college with a lot of people from a lot of different places, probably far from them too. Goodbye friends.
3. It's not unlikely your job brings you to a city, and US cities are notorious for having the tall downtown where most city life exists and the quiet, expansive suburbs. You're young so you move downtown. Time to make new friends.
4. Downtown will be expensive, apartments will be for rent not sale, so you'll be moving about constantly. You never form meaningful connections with neighbors.
This point is interesting in that is, I feel, why people seem more glued to their phones today, interact with people around them less, and overall annoy each-other more. Their real friends are not necessarily the people around them: they live on your phone. You don't make an effort to meet people around you cause you might never see them again. Not even your neighbors: your lease is for just one year.
5. You eventually settle down and move to the suburbs. Time to make new friends. Everyone wants the lawn, but this also means houses are far between. How many people can your really interact with? Moreover, American suburbs are just that: houses. No one walks on the streets of suburbs, so meeting, bumping into and interacting with neighbors takes effort. All the time you spend shopping, working, having fun-- it's probably going to be around strangers downtown, not your neighbors.
6. You have kids, and your kids leave for college. They're not going to move back home, or to your neighborhood. They're going to go down the same path-- you better remind them to call.
7. You get very old, and then get put in a elderly home. Again, instability, and you probably have to find and make new friends.
A lot about this progression seems relatively normal but are unusual where I'm from.
