system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is Japan's Declining Population viewed as a concern when geographically they are a small nation, and overpopulation is something that seems a like a global issue?
gpt
Draw a triangle and split it in half across. At the top, draw "the elderly" and at the bottom draw "the young". This is the ideal society wherein the young people help to support the needs of the elderly. Since there are more young people than elderly, not much burden is placed on young people to support and care for their elderly parents/relatives.

Draw another triangle, but upside down. This time, put young people at the tipped end and elderly at the top. You see that this triangle is unstable. There are too little young people to care for the needs of the elderly population. This is the current issue that a declining birth rate countries are facing.

But that's for a personal level.

On a state and economical level, there will be less people that will be able to work as time goes on, as many elderly become too sick to work/retire. As a result, there will also naturally be less consumption by the population. Thus, this means a general loss in government revenue since less people are paying taxes. The economy might also stagnate due to a lack of business in the country. 

As one commentor has pointed out earlier, it is a sign of a dying country. This is true; we have an excessive burden of young people in caring for the elderly, and a stagnating economy. This is why it's a big problem.