Reading through the thoughts here, I think there is a large emotional component of parenting that's being ignored out of naivete. And it's not just that there are hormones and biological mechanisms that reprogram parents brain in the run-up to the birth (though there is that, and probably similar mechanisms that we don't know of).
But ignoring that, as you alter your life for a newborn and then care for the child, your internal, life-narrative starts to define your self by that child. It's utterly unlike getting a hamster one day and similar but infinitely more than getting a dog. (But also, like not because the average response isn't the same as an individual response. So of course there are individuals who don't develop this attachment for various reasons.)