Lots of varied answers. Thanks everybody.
I think that most people have a parenting instinct and in some it's probably stronger than in others.
To those without children I'll say that being a hands-on parent day to day is a very BIG challenge. But it's very rewarding too. You get to see a little person grow mentally and physically and hopefully the legacy left with them is a positive one. Of course, chosing not to have children is OK too, because you direct your energies to other rewarding pursuits.
Anyone else care to share your thoughts?