I don't want kids right now. I don't believe that I will ever want kids, but I know that things can change my mind about other things. I have had surgery to prevent me from ever birthing children. Voluntary and much-wanted surgery. But I do know that if I decide down the line that I want kids, I would much rather adopt. I have so many reasons for not wanting children in general, or not wanting them of my own blood, at the least. Most have been listed here already.
But it really doesn't matter. Who cares why someone else doesn't want kids? The fact is that they don't. There will be no lives ruined by the decision to not have kids. Some people will make up a reason just to have one beyond "I don't want them." Not having the money, while a perfectly valid reason, is one that many people use. Yes, this can change, overpopulation could, theoretically, change, all these reasons can change. But it is a decision that belongs to each person, and no one should forcefully try to change their mind about it.
So you think that someone's reason not to have kids is selfish - if that were actually the case, do you want a selfish person having a kid? Or immature, or any of the 'bad' things people say about those that decide against kids?
There's also no reason to get on anyone for their dislike of children. I like them up until the point that they can speak sentences; this is where I get nervous and can no longer deal with them confidently. That's my own decision, I certainly don't hate kids older than that, I just don't want to be around them, if I can avoid it.