system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does Japan have such huge cities?

I was on Google Earth the other day and when I was looking at Japan I was astounded as to how much city there was. There were loads of them and they were huge, incomparable to American or European cities. Why is this?
gpt
Population density.  Like most places, Japan has been urbanizing since before the second World War.  Good health care combined with historic longevity pushes the population up and up.  Being an island, there is limited opportunity to build new cities, so the existing ones creep outwards and upwards - there are some really cool skyscrapers in Tokyo.