Is there some inherent instinct to multiply?
Why do we have kids? And why so many?
There is no possible chance that humanity will go extinct. The population is expanding.
So what is it that drive us to have children?
It just seems like it's something we're "supposed" to do.
It's part of life.
Are people who don't have children somehow deficient?
Is breeding one of our very few instincts?
In my experience...
Feelings of obligation.
Simple desire for children.
Fear of being alone in old age.
That covers pretty much everything I've ever heard of.
People don't really have an "instinct to breed," so much. We don't really need one. We have a desire for sex, and complex societies capable of influencing the behavior of most people. That's enough to keep most people reproducing.
In fact, the level of "nurturing" of parents varies considerably across cultures. People who either live in very poor cultures, or in very primitive ones where too much reproduction would collapse the tribe, tend to be very stoic and tactical about reproducing. They don't see all reproduction as good, the way those of us from richer countries tend to.
I think for the majority of people, they don't put a lot of thought into it. They just sort of assume from an early age that they will have children, because society presents it as an inevitable consequence of adulthood, and they simply never had a reason to question that. This is somewhat bolstered by the fact that about half of people become parents by accident. I've met a lot of people who talk about having kids with a sort of nonchalant resignation, and when I ask them why they assume they'll have kids when they sound so unenthusiastic about it, they just say, "Well, everyone has kids."
Some people do really pine for children. They often work in child-centered professions until then, or after. They're just good with kids, "get" kids, and want kids. In my experience, a lot of these people either help kids not their own, or adopt kids in addition to having their own. They tend to produce their own, simply because it's kind of the easiest way to do it. But they're more likely to adopt, because they just inherently value kids, rather than doing it being it's just "what you do." Perhaps these kinds of people have the closest thing that exists to an "instinct" to reproduce, or at least an instinct to contribute to children.
And for a minority of people, it's almost kind of a nerosis: people who pin all of their self-worth on being parents, like a sort of status symbol. These are the people who just marry the first healthy person they meet, or have a complete break down if they haven't reproduced by a certain age, or spend themselves into $50,000 of debt and eat rice every day because of treatments trying to force their misfiring reproductive organs to work, rather than adopting (and somehow still think they'll be able to afford the kid if they ever succeed). People who care more about the status of "parent" than they do about what kind of parent they're going to be. These are the ones I get the least.
Personally, none of these reasons have ever been very compelling to me. I get much more gushy over kittens than babies. Perhaps I'm just missing that wire.
Since I don't have much of an emotional reaction to the idea of having kids, I think about it in a pretty technical way. And there's really nothing about the lifestyle of being a parent that appeals to me in the least.