An aging population, a high infertility, and a lack of food and water.
Answer:

Well, you can take the example of different genres of music. One person might like pop while others might be more interested in country, jazz, hip hop etc. So, what seems good for one might not seem good for others due to their tastes in different things.

The Chinese failed to foresee that the European expansion would rival their rise because the Chinese did not see the importance of voyages across oceans. The Europeans were out to discover new lands, resources and colonies. The Chinese were thinking about how to bring together their nomads.
The use of civil service exams