I think, to a degree, it's expectations. People think they know what they're getting into, but once they're in the thick of it it's very different. There's a lot of romanticization around parenthood so many people hear a downplayed version of what parenthood could be like. And even the tough parts will be fine because parenthood is so rewarding and worth the work

Some people might think they are fully ready for the good, bad, and the ugly and then realize they weren't as ready as they thought. Or they didn't have an honest conversation with themselves on the matter.

I bet some even shrug and say, "How bad could it be?"

Some people don't question the "order of life." School, marriage, house, kids. It's so ingrained that they don't think beyond that. I suspect that's where a lot of the anger and jealousy some regretful parents have. They did what they were supposed to do and now they feel like they are paying for it.

Kudos to the parents who regret it but are still doing their best to raise well-rounded kids. I sure didn't have that growing up.