There's multiple reasons as to why the human population is growing exponentially, some of which are:
- lack of education
The lack of education, especially in the less developed countries, results in very young marriages, teenage marriages, where the two teenagers imitate the behavior of the elderly people in their family and surroundings.
- pride and tradition
There's still lot of parts in the world where having as many kids as possible is considered as tradition, or as the biggest pride of the family, though the poverty and the living conditions for those kids are rarely taken into consideration.
- irresponsibility
In the less developed countries, where the birthrates are the highest, very often the kids are married by their parents at a very young age, they have to work from early teenage years, they live on the verge of existence as usually the families are poor but have lot of kids.