I'm childfree by choice-and honestly, whilst I could easily list off a multitude of reasons why I don't want kids if I had to justify myself (I don't) it all boils down to one thing: I don't want them and part of that is as instinctual as it is logical.
And I think some of the same thing for the reasons why people want kids. It's not always based in "logic." It's based in feelings and instincts and that is as valid as logic or can be.
A lot of my "reasons" for not having kids come down to freedom. I want my lazy Sundays, reading my books on the couch and to be able to come and go when I please. I want to go on holidays that aren't themed around children. I want to live my life on my terms.
And if that is selfish-I don't care. We all get one life and why shouldn't we be selfish with it? This is a decision that impacts no one but the person making it so if that decision is "selfish" so be it. There's no cupboard of souls, waiting to be born and by some of us opting out, those souls are denied the opportunity of life.
Kids and parenting isn't for everyone. It's hard work-and my DB has a DS and whilst I love the kid to pieces and adore spending time with him-I know I couldn't do it on a full time basis. It's just not for me and I don't see any shame in admitting that parenting is not one of my capabilities. I think it's better to be honest now and make the decision not to-than to have a kid and just see what happens.
Also, with justifying why you do or don't want kids-it doesn't make sense. For some people, no justification, no amount of reasons for not having kids will ever be sufficient. They'll always find holes in your reasons. Ways that you can still live the life you want with kids attached.
Similarly, if a parent or someone who wants kids tries to justify their reasons for having kids to someone who doesn't want them-no reason will ever be deemed sufficient or good enough.
This is why we just shouldn't bother to justify our choices to people. It's the path to nowhere. If someone presses you for your reasons-tell them to mind their own.