It's branched from how I think about TFR. To merge,

- the common sentiment of "child raising is too expensive"

- the reality that wealth has drastically gone up

I think: okay, it must feel expensive for some reason. Probably because the work involved, despite not changing too much in an absolute sense, is relatively much pricier compared to all modern cheap sources of happiness.

Then, this notion of the cost-of-fun is easily transferred to general socialization & the loneliness problem.