My 2 cents is this;
I think there is enormous societal pressure brought to bear on marrieds to have children. The truth is that many child bearing couples lives have been enriched by having children, and they think that only by having children can you begin to understand the absolute joy they themselves feel in loving another so unconditonally. So the logic would be that they feel you don't know what your missing if you choose not to reproduce, bringing about the "you'll change your mind" comments.
Also, as a species we are hardwired to reproduce, it's in our nature. So when the masses are happily following the biological imperative to go forth and multiply it's also natural to wonder about those who do not.
I am a labor and delivery nurse so I see every spectrum of the childbearing woman . From the 13 year old who is still a child herself to the settled couple who begin child bearing after 40. I also see women who are desperate to concieve who are in agony because they remain childless, they feel somehow less than, because thier bodies cannot do what they percieve every other woman takes for granted, they ability to have a child.
What I see most of all unfortunately, is young 20 somethings, having kids without any thought to the impact these new people are going to have on their lives.
You need to pass a test to aquire a liscence to drive a car, to prove to others that your capable and safe behind the wheel. There is no skill testing question required to become a parent, and often it occurs without forthought and purpose.
I think that the majority of people who decide NOT to have childen give the decision a great deal of thought and don't take their choice lightly. I only wish people were so deliberate in the decision to have kids rather then just kinda going along with the "next logical step in the circle of life" mindset.
In case you were wondering I'm 32 without children...
Personally, In my case if I were going to have a child I would want to be an excellent parent and I'm just not willing to put in the work required to be one. Raising children into purposeful and productive and nice human beings takes a lot of effort. Call me selfish if you like... I prefer sensible.
We've all seen children who have NOT had the effort put in haven't we?
Sorry, got long winded, my 2 cents is now 4...
Have a good day all!