It depends. In countries with a lack of social security and pensions people tend to look on their children as providers.
If infant mortality is high then they will likely decide to have many children to make their bet more safe. If they have girls and the local custom centers around having sons then that too might work as a reason to get more children, and the reverse will be true in places where large dowries are paid for daughters.
Having many children is also seen as a sign of affluence or blessing.
Lack of education and contraception are factors in the developed world too, people that have lower levels of education tend to have their first children at a younger age and tend to have more of them as well (both due to starting younger and due to not being able to accurately gauge the impact of having children on their lives).
I'm sorry you got downvoted, overpopulation (or simply the problem of how to match our numbers to the available resources without drastic measures or apocalyptic events) is a subject that requires one to face some harsh realities and ask some very tough questions.
