system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we talk about the Earth being overpopulated, and not individual countries? If X Country has more babies than they could sustain, why is this a concern for first world countries with significantly lower birth rates?
gpt
Because its a global issue, as much as we try to maintain national sovereignty everything is connected such that a problem somewhere else is a problem at home as well. 

High populations require resources in terms of energy, food, water and space and that increased demand affects total supply. Water and food demands are local problems until they run low then it becomes a regional problem as famine kicks in and forces migration. Migration moves the problem elsewhere and you start getting people who are extremely vulnerable away from home with out enough to sustain themselves this can politically destabilise an area as people without anything are vulnerable to violence.

Now what does that have to do with anything? Surely what happens in Africa is no concern of a low birth rate country say in Europe but that's not true. In order to find safety away from.violence and starvation you get people trying to enter safe countries as refugees either 'legally' or 'Illegally' or more accurately documented and undocumented. The problem here is now you have vulnerable people with documents waiting in camps to be processed and sent to safety and the camps aren't really all that safe. You also have people trying to bypass it for whatever reasons risking their life trying to cross the Mediterranean sea to Italy or going east to Australia via stepping countries.

Now the most singular important reason in this long rambling post is empathy for human suffering. People generally don't want other people to suffer which means generally they'll try to help.