Reason 1: The population of the world is increasing too fast.
The population of the world has increased by a lot and is suspected to increase by a lot more by the years. In the US the amount of people in just ten year has changed very surprisingly. The 2010 Census reported 308.7 million people in the United States, a 9.7 percent increase from the Census 2000 population of 281.4 million.
Reason 2: We will eventually run out of space to put all of our people on Earth.
Including land and water of the whole Earth counting the habitable places the whole Earth is 510,000,000 square kilometers. Pretend that the people population is exactly 7,000,000, this is the amount of people that would take up each: 13.7 per km2.
Reason 3: We have increased in size too rapidly.
In just about 10 or more or less years we grow over one billion. Eventually Earth would just be covered in people. Think about jobs what would we do if we had so many people we would have a person for the job which might leave even more people jobless.