People aren't having enough sex. Consequently, Japan isn't having nearly enough kids.

A reported 1,008,000 babies were born in Japan in 2015, while 1,302,000 people died. That's a net loss of almost 300,000 humans. 

Compare that to Sweden, which is growing and has a high fertility rate for a developed country. In 2014, 110,907 babies were born and 88,976 people died, making for a gain of 21,931 people. 

The downward and upward trends speak to the health of a country. In the case of Japan, a country with one of the world's largest economies, the population is set for a 'demographic time bomb' to explode.