Science and biology in particular tells us that life forms opt to reproduce (allegedly) for the sake of keeping their genes (a study showed that some species males tend to kill the offspring of a dominant member that has been defeated to make the females in that group available for mating and thus the prevailing males would pass their genes through mating.
The 100 million dollar question is that; we understand why animal do it, but what about mankind? ask anyone around you withkids: why did you opt to have kids? The answer will depends on how sophisticated the person is. Most will answer the traditional instinctual answer (I want someone to carry my name/my genes) but reality; for humans and germs alike says that every reproductionattempt is done out of fear of death, we do what we do thinking that we will evade death and fulfil our futile search for immortality somehow in our children; our copies, our extensions.