Our minds -- whose involuntary emotional reactions drive our behavior and impart to us many of our basic values -- are evolved. Evolution tinkers, reuses, never redesigns or reorganizes. So instead of pushing us toward the single, monolithic goal of having children, we experience constant nudging to do things that are conducive to reproductive success. Support yourself. Be well-liked. Have sex. Love your children. Achieve status. In slogan form: evolution's instrumental values are humanity's terminal values.

So to answer your question, we act to make ourselves happy, and what makes us happy approximates what is good for the next generation. There are gaps, though. Some people to focus on their careers to the exclusion of family, and contraception doesn't engender the same reaction as eating rancid meat. I tend to think this is a good thing, but I'm status-quo biased and my values are largely a product of the current arrangement, so that's unsurprising.