Before I became a parent I cared about children in general, intellectually, in the same way that I care about people in general: a general sense that I want good things to happen to them. Over the first few days of Lily's life, however, I noticed I cared for her deeply and emotionally, where her wellbeing was incredibly important to me, and she was far more cuddly and interesting than I expected. Over the next few months, I realized that my disposition toward children in general had also changed, and while I didn't feel as strongly as toward my own, they were now similarly far more fun, interesting, and precious than they had been. I also started having strong emotional reactions to reading about harm to children. Paul Graham writes about a similar experience, though I think one a bit more sudden than my own:

Partly, and I won't deny it, this is because of serious chemical changes that happened almost instantly when our first child was born. It was like someone flipped a switch. I suddenly felt protective not just toward our child, but toward all children.

And Maia commenting on a post of Julia's:

For me it was like a switch flipped when I gave birth. The mental change felt similar to the mental change I went through during puberty: just adding a completely new biological drive I didn't have before. I actually didn't find babies very cute or interesting at all before, but I found my baby extremely so immediately

In deciding to have kids this is a very difficult factor to consider, because it doesn't happen to everyone. I have a vague impression that it is a bit more common in men than women, perhaps because women understandably have a higher threshold for deciding to have kids, but there are definitely men who expect it to happen and are dismayed when it doesn't and women who are surprised at how much their feelings toward children change.

Getting better at predicting this transformation seems valuable: since becoming a parent is a lifelong decision, the value of information is really high. Do people tend to have similar experiences to their own parents? Is it correlated with any other aspects of personality? Can your friends or other people who know you well predict this? Can it be triggered, if it's going to, by something less permanent than becoming a parent or is the permanence a key component? Is it something that may or may not happen, and when it happens it is a similar magnitude for everyone, or is it something where most people change a bit on becoming a parent, anywhere along a range from "imperceptibly" to "enormously"? Do we know how prevalent these changes are?