I always thought people had kids so as to make their life more meaningful in the following manner: they know that when they die they will leave something about themselves: their genes, and perhaps some off their values. But what I've read seems to differ from this idea: people have kids because they like themselves so much they want to see mini versions of themselves together in a child in this life. So what is the truth?
You can certainly make an evolutionary argument: species that do not have a strong drive to produce offspring will become extinct quickly, species that want offspring will persist. Since humans are not extinct (yet) it stands to reason that, as a species, we do have a strong genetic impetus to procreate. It's baked into basic human nature.
The rewards are mostly emotional: raising kids cost huge amounts of money, it's an enormous amount of work, a massive time sink and in general a pain in the neck. It's really hard to justify other than through emotional rewards.
I have three, love them to death, and it's the best thing that ever happened to me (emotionally, not financially, that is :-)