The past is a foreign country. I do not believe "quality of life" in economic terms is a sufficient explanation. The simple Darwinian fact is that, if your culture did not value reproduction sufficiently, through whatever means, it would not have survived to the present. Remember, infanticide was widely practiced historically, and while not as convenient as birth control, provides a significant fraction of the same "benefit".

I do find it a bit ironic that "free child slave labor" is considered a better reason to have kids these days than, say, ancestor worship or nietzschean vitalism.