Lots of theories here. Industrialization. Wealth. Education. First world transformation. Kids' expense.

All of these are second-round of reasons.

Primary reason: Materialistic wealth or wealth in general is preferred over human contact.

Effect: People connection drops. Community drops. Independence and Individualism prosper.

Secondary effect: The seeds for family development (community, human connection, village camaraderie) go missing. Growing a family now requires artificial support. When family members grow up, their time is now spread across materialisms and career development. Career development goes up and takes priority. Wealth acquisition takes priority. Except everyone is doing this so basic needs have to be fulfilled with limited resources. All prices are now going up. What was the point of everyone working now? Wealth acquisition. hmm.

Tertiary Effect: Huge workforce looking for work. Wages diminish because supply went up. Businesses prosper. Market caps go up. Business becomes easier. Dominance becomes easier.

4th Effect: Debt goes up to fulfill materialistic quests. Interest locks in people into a debt that grows over time even if the supply of money does not go up. Now people are perpetually looking to complete paying off their debt. And they will perpetually need to work. And worker supply perpetually increases. Freedoms go away. Wealth centers on to certain people. They take over media, entertainment, recreation, and tourism. We end up with a tale of two worlds.

Edit: Before the primary reason goes into effect, I will acknowledge industrialization improved people's access to wealth and materialism. And that replaced human connection.