I can't think of any one thing in childhood, but I have never wanted children. While one sister, in particular, only dreamed of growing up and being a mommy, and many of our friends did, too, I considered such an idea for me as horrific.
Doctors tell me that I'd regret having my tubes tied but I'd just assume haul them out as I don't plan on ever using them.
As an adult, my reasons for not wanting a child formed. Being a mother is the highest calling a woman can have and I shudder at that responsibility. It's not that I don't like responsibility; I just don't want to be responsible for giving birth to another Osama bin Laden or Adolf Hitler. Even the best upbringings churn out evil adults, and god forbid that should be me! (Yes, I understand that raising a monster is not a given, but what if? I couldn't handle that.) My family has genetic medical issues that I can't imagine passing on to another human being.
When I held my oldest niece for the first time, I felt, for the first and only time, a twinge of wanting a baby of my own. I've got my nieces and nephew; I can leave my mark on the world through them. I don't need my own children!