I think that the human mind tends to forget the negative and focus on the positive. I have undergone various surgeries and remember blurry what happened and that "it wasn't so bad". I also know moms who think that "next time will be easier" and the lighting won't strike twice. And ofc social pressure for a second kid. I am an only child and growing up I witnessed my parents being constantly told "why not a second one, she is so cute" or "you should start the tries for a second baby, siblings shouldn't be too far apart" or "isn't your child lonely without a sister/brother"? Could be more reasons ofc that I cannot think of since I am not a parent.