I think there are strong instincts in a lot of people, maybe in the sense that they want to nurture and care for someone else. I personally do not want and will never have children, but I do want to nurture animals. It seems that some people don't have this instinct at all, though. There's also the curiosity of what children will look like as well as wanting to pass down the family name and genes. I don't think those are as strong as the nurturing aspect, though.