People fear what the media and governments tell them to fear, since the media and government has told the masses to fear overpopulation that is what they fear. What they haven't been told is that when taking the entire world into consideration, there is more than enough livable space for every human in the world to have their own piece of land. There is still more than enough land left over for farms to provide food. We do not have a shortage, we create a shortage.
Humans believe that we can just go about things the way we have always been without repercussions for our actions. But we will learn in the next 100 years if even that, that there are consequences to our actions. Part of that is overpopulation and depleting our resources. And when there are too many people on the Earth for it to handle then what are we going to do? Take only a select few and start over on the moon?
No, overpopulation can be a serious concern. If there are too many people to be sustained by the resources around them they would either have to expand to other areas or die. Due to the fact that people couldn't just pour over into other nations outside of the one they live in, overpopulation could be a serious problem.
The more people we have, the more people fight for resources. Some say, it is good to have so many people because you will have more people that will invent things. They will move our society faster in making improvements. The problem is they leave to many people behind and those people end up weighing our society down because no one is willing to teach them.
History has shown us that overpopulation truly is something to be feared. It can lead to health problems or a lack of resources. One of the diseases we strongly associate with this, for example is the Bubonic Plague. But even in current times, we've seen problems with areas being overpopulated to a degree that there is a lack of resources, such as water. So overpopulation is definitely something that can cause health problems.