Once, I lived in a small rural college town (land grant - the U of Idaho). I could walk or bike anywhere I needed to go. Cars were required in practice if leaving the city.
I used to live in outer ring Seattle proper (Roosevelt/Ravenna n'hood junction). It was a drive to the store, but an easy walk to the bus. Buses got me where I needed to go, mostly - if I needed to go to specific zones. Most of the surrounding blocks were SFH, with a business corridor nearby.
Later on, I lived just outside of the 'urban core' of Seattle( Lower Queen Anne ). This was a full mixed use zone. I could walk to daily needs, but busing was frustratingly constrained to specific target areas (Downtown, Capitol Hill) in practice.
In both of the Seattle scenarios, I lived in an apartment building. Neighbors were deeply uninterested in talking to each other, as a rule. While I could get to know some of the baristas, in general, social behavior was limited to organized clubs, not place-based friendships (unlike the usual predictions). This is a known thing about Seattle.
Now I live in a smaller city (Spokane), in a 1900-1920 era neighborhood, and I can walk to my daily needs still. I live here in a SFH. The local culture is much more welcoming than Seattle.
One of the angles that I personally ran into and found frustrating is that apartment buildings _as they are built_ are not set up for families with children. Things from outside decking (painful to fall on) to railings (easy to fall over for climbing kids), lack of adequate play space, etc. LQA in particular was _remarkably_ unfriendly to children, and most families I encountered in the daycare seemed to be on the road to moving out - as I did as well. Part and parcel of increased density thinking - for it to be successful as a long term strategy - has to incorporate design elements that address the full human lifecycle and experience: children are a pretty fundamental part of that, that is _left out_ of much (most?) urban buildouts in the past 1-2 decades.
There are other issues in density - if you hold crime, drug abuse, and homelessness constant per capita, when you have more people per sq mile, you have more social issues per square mile, and _that_ seems to be a problem the United States hasn't solved in its cities at all.