The reason for overpopulation is simple, really. The world is overpopulated because people are generally unwilling to control their reproduction. If you marry and have only two children, and each of them has two children, and so on, we have exponential growth. However, there is more to the story than just that. There are many complexities within that simple reason, having to do with culture, religion, politics, medicine, technology, and geography. Let's look at how these factors play out in two different countries.
In the United States, which is a constitutional democracy, we are guaranteed reproductive rights, meaning that we have the right to choose to have children and the right to choose to not have children. This means that any responsibility for population control rests primarily with the individual, not with the state. It would be easy to say that we should all simply have fewer children, but that is where these complexities come in. There are cultural and/or religious reasons that some people have as many children as they are able. And so, while there are people who have no children at all, for whatever their reasons, these are not enough to prevent overpopulation because of the exponential nature of population growth. But, you might say, we have plenty of land and natural resources to expand. A very small percentage of the land in the United States is settled. Well, that is true, but even with this, overpopulation is a problem. How much space we have is not the issue. How we will feed, house, employ, and generally care for an increased population is the issue. If growth is unlimited, can we sustain it without destroying the environment permanently? Technology has been a friend and a foe, allowing us to settle people in areas previously considered unsuitable habitats for man, allowing us to tap more resources and use them more efficiently, but also lulling us into the false belief that technology will always be able to solve all problems. In a way, technology has become our new god. Similarly, medical advances have created, in the United States, at least, a situation where the diseases which previously decimated or culled populations no longer do so. We have largely conquered cholera, smallpox, bubonic plague, and polio, for example. Small outbreaks occur, but they are quickly contained and no longer become epidemics.
On the other hand, in China, the government is able to control population overgrowth. It does so by allowing most people to have just one child. This probably seems like a cruel policy, and certainly it causes familyies pain, as well as societal problems such as insufficient numbers of children to care for ailing families. However, if the Chinese government did nothing, there would be nothing left at all for succeeding generations.
If individual, national, or international efforts are not made, life as we know it will be unsupportable in a future that is closer than we think. In the link I have provided, there is a counter in the upper left corner that shows how quickly the population of the world is growing. Presently, we have nearly 7 billion people. Watch the counter for just a minute. You will be shocked.