It's everyone's right to decide whether or not to have children and they should not be condemned for that choice. When it comes to grandchildren, the choice is made by someone else but you shouldn't be condemned for your reaction to that choice, whether it is positive or negative. In fact, if you don't have strong maternal feelings, it is probably better to have the courage to remain childless than to submit to pressure and become a mother when you don't want to be. Similarly, if you don't want to be a grandparent, you are lucky if it doesn't happen - for your own sake and for the children you would have grandparented.
Having said that, I think it is rather rude to suggest that grandparents monopolise conversations by droning on about their grandkids. A few, maybe, but most people just talk about them a bit because they are an important part of life. I have noticed that this sort of criticism is usually aimed at women - for instance, mums will be accused of talking about nothing but their children, whereas their husbands can witter on for hours about jobs, cars and so on, without attracting similar criticism.
I wonder whether the reaction of "perhaps they'll change their minds" is prompted more by embarrassment than pity. If you've been talking happily about something and you are then told that it is not part of your listener's life, you would probably feel very awkward and say something to try to defuse the situation. Of course, in the OP's case, it is the last thing she wants to hear, but I imagine it is usually well-meant.
I do consider myself lucky in that both my DDs have children, though I feel sorry for myself when I have to face the fact that 4 out of our 6 DGC live nearly 4,000 miles away. Each one has enriched my life and I am thankful to have 2 DGDs living under our roof. They are a huge part of our lives. I don't expect everyone to want to be a grandparent, and I'm sure that some people have no wish to have any more children in their lives, but I think there has to be tolerance on both sides, between the haves and the have-nots.