Why do some people regret not having children?

There was a time when I was completey self-absorbed & didn't want children - ever! But I changed. Now I see children & smile. I am not threatened by them as before but see them as mankinds greatest faith in himself. If you took measures to ensure you would never have children it most likely will be a regret. We all change down the line & children, being the natural result of that change, become quite important to us. They are life - extended. They are joy on earth. They are us & we are fascinated by that!