Why are grandchildren considered the "reward" for raising children?

I've never understood this way of thinking. You pour your blood, sweat, and tears into having children of your own, only to be rewarded with... more children?? Can someone explain this way of thinking to me?

And I understand that grandchildren are often seen as all of the fun of having kids without all of the work, but... When I told my mom that I'd get an abortion if I ever got pregnant, she got all defensive and said, "Why?? I'd raise them until you were ready for them. What's the big deal?" So, at least for her, she's totally content to not only do fun stuff with her hypothetical grandchildren, but take care of all of the boring child-rearing stuff as well. What is going through her mind when she says things like this?