I think this is also in part (in addition to the immigration) because the US overall is a pretty happy country, compared to places like Korea or Japan. It is still possible to afford to have children, though increasingly difficult.
A "happy country"? I'm not sure what that's supposed to mean. In terms of the cost of having children, though, it is far more affordable in Japan than the US. Almost all childbirth costs are covered by the government, and free nursery school is also provided to all.
