Ever been to a major city and felt a little crowded? You're not alone. There are too many humans living on this planet, and the population is increasing at an exponential rate. There is only so much space, so much food, and so many natural resources. We're already probably at stretching point. Much more than this, and we won't be able to cope. It will be the developing nations who suffer in that scenario more than anybody else.
Consider this statistic: There have been human beings living on this planet for thousands of years, but the population of Earth has doubled from 3.5bn to 7bn in just the last fifty years. People are living longer, and better medical care means fewer people die, however in turn that leads to overpopulation. That's more consumption, more pollution, more famine, and more disease.