I think it's social pressure in some cultures, but even in those (sub)cultures which don't have the explicit pressure to have kids, people have a drive to procreate. I cannot tell you how many couples I know who decided they would not have kids, or would only have one, and at a certain point one partner (usually a woman in their late 30s or early 40s if I am being honest) decided this was the most important thing and no amount of money or career setbacks or relationship turmoil wouldn stand in the way.