It causes lots of problems because many of our systems have been set up in times of growth and won't function without it. Take for example "pay-as-you-go" pension systems in much of Europe. Here, you don't fund your own retirement, but the current working population funds the retirement of the current retirement population. This works great when you have population growth, because you can have e.g. five working people fund one person's retirement. If the ratio moves closer to 1:1 or worse, this becomes a lot more challenging.

Of course, population growth cannot continue forever, so we will have to figure this out anyway. Still, for any individual country, the smart move seems to be to stave it off for as long as possible, observe how other countries deal with it, and then implement the solutions that have actually worked for others yourself.