Having children becomes more of an option as technology and society progresses to a state where children are less of an assett and more of a liability. I remember learning this in basic sociology in college. Back in the 19th and early 20th centuries in America, for example, more children meant more labor, meaning the family was better taken care of. Moreover, as many diseases were still rampant back then (yellow fever, polio, etc.), there was a much higher death rate among children, so it was better (and therefore common) for families to have 8 to 15 children or more. Nowadays, society is vastly different--we are much more adept (and expected) at taking care of ourselves in a financial sense, and with child labor laws in place, the reasons to have children have changed to more lofty (yet arguably somewhat conservative) ideals such as carrying on the family name, leaving a legacy, or what have you.
Ultimately, the primary reason to have children will always be, of course, the innate desire to perpetuate the species. It's odd, though, that so many of us can, even in such a current age as ours (where we are still very underdeveloped), deny that primal urge to procreate...at least as far as producing actual children goes.
