According to the traditional concept, children are assets and blessings. In agriculture society, the number of children is also related to the manpower of a family. The more the children, the greater the manpower. Besides, in the past, the death rate of babies and small kids are very high too.
However, nowadays, the concept has been changed. After the explosion of population after the war time, many countries especially those developing countries are experience of insufficient resources - food, space, education, .... So, some people raise the concept of birth control. One the other hand, the developed country, people life styles are different. People are looking for high living standard. They enjoy more freedom whereas, if they have children, there are more restrictions in the way of living. Therefore, some young couples even plan not to have any child.
Another consideration is how to raise up children. Parents prefer to have less children but provide them with better resources and let them have better development. When parents try to contribute more for their children, they are also expecting higher return. Parents always hope when their children grown up, they are competent, responsible, have good lives, ... Thus, creating a lot of pressure for both the parents and the children. In order to bear less burden, some couples decide to have less children.
Furthermore, the working conditions are different from the past. Overtime work, unstable job, ... are also constraining some young couples dare not to have children.
In the past, people consider marriage as a life time commitment. Nowadays, people do not have sufficient faith that marriage last long. Some even sign agreement if devoice occured, how to handle the possessions... Those do not have faith in their marriage, of course, dare not to have child. Breakage of marriage will affect the next generation greatly.
I think, there are much more reasons other than those listed above.