The quality of living is high in Japan, due to a lack of poverty in most major urban areas. Look at Tokyo or Kyoto. Most everyone, save for maybe a few people, live in middle class housing. Slums and low-income areas (excluding rural areas), for all intents and purposes, basically don't exist in Japan. As a result, the high cost of living is a factor in most people's decision on whether or not to have children. This is also probably combined with the fact that the government isn't providing any sort of incentive for people to have more children, so to the Japanese, it's worth saving the extra money.