Some people do not want children because they know they are a huge responsibility and do not want that huge responsibility.
At first I thought I would never want to have any children.
But later on in life I decided I did and after my first kid I found out that although raising kids was hard work that it was not as bad as I thought.
I have 4 kids now and 2 of them are almost grown up now and I still have a 2 year old boy who is the sweetest thing ever.
His smiles and laughter make my day much brighter even when I feel frustrated at something he's done.
Children also cost a lot and require a lot of time and some people are just not meant to be parents.
If someone doesn't want kids then that is okay as well.