It is because raising children is a selfless act. The more children a couple can handle, the more selfless they are. A lot of folks stop at two because with that second one you realize you are spending all your time pleasing two little tyrants.
However, one of the great secrets of life that few people act on is that you are happiest when you aren't getting your own way. People who spend their time trying to make themselves happy are the most unhappy and depressed.
Children are ultimately the most joyful thing you can have in your life because they give you a mission and a purpose outside of yourself.