What is selfish about not wanting kids?

I was talking to my coworker about having kids and how my generation doesn’t want them. He said he thinks people who don’t want kids are selfish and I said I think it’s actually quite selfless because they know that they’re not fit to be a parent. And he said I guess that’s true but if you said anything else I would have disagreed with you and not continued this conversation. I just don’t understand what is so selfish about not having kids when it absolutely does not concern anyone else’s life, but it’s the number one thing people say when you say you don’t want them

Edit: in conclusion, not having kids is selfless because we are selfish.