I believe the greatest factor is community. In my experience wealthier people, and wealthier areas, tend to have less community and weaker interpersonal bonds because they do not depend on one another to the same extent that poorer people do.

When your neighbor needs to borrow a tool, you need to sleep at a friend’s place, or you give a friend a ride to work you’re building relationships. The web of relationships between all the neighbors in a community forms a culture.

When people become wealthier they don’t need to borrow tools because they can buy their own, they don’t need to crash at a friend’s place when they can stay in a hotel, and they don’t need a ride to work if they have their own transportation.

In my experience some of the isolating effects of wealth accumulation can be mitigated with infrastructure that increases the inter-dependence, trust, and fraternity between neighbors. A few examples are walkable cities, cooperative organizations, social clubs, public parks, etc…