The child's soul, whole, grows... whilst your damaged soul whithers. Evidence of this can be seen by the zombie like appearance new parents have, still coping with this horrible loss.
As time passes, you become more accustomed to this loss, and it becomes like Stockholm syndrome. You eventually like it, embrace it, concoct wild fantasies about how great the experience is. It is your own mind defending you from the horror, and more importantly, you wish this same fate upon others.
Don't believe them. Don't trust them! Keep, cherish, protect your soul.
I am not extrapolating my experience to everyone. I'm pointing out that the vast majority of people who have children find it a positive experience.
Lastly, I'm not dismissing their feelings, I am reassuring them that they are very common indeed.
It’s like buying a 1M dollar without seeing it and having someone tell you “you’ll love it when you cross the threshold because I did with my house!”
Not saying it didn’t happen to you or your experience is invalid. But extrapolating your experience to everyone else feels dismissive about the doubts they are expressing.