I know my Mum (who was quite a religious person, born 1930s) thought it was almost a duty to replace yourself - so if you were a couple you should at least have 2 children. And, i suppose, looking at things from the point of population, that is very logical.
And, i suppose, if you very much enjoy your own life, then it could be viewed as a bit selfish not wanting to give that opportunity to another person by opting not to bring anyone else into the world to enjoy it as you have.
So - there's 2 possible views.
And - in a literal sense - it is selfish if you only have yourself to think about.
Of course, many childless people are caring for parents or vulnerable family members but for those that aren't then they might not need to put other people first very often at all.
They literally are able to put their own needs first at all times without needing to consider how their decisions impact others.
Can't think of any other reasons. And those aren't necessarily my views. Just answering your question.