Snack, Anyone?

My grandson Griffin is nine months old, and has spontaneously started sharing. Whatever he's holding, he offers to anyone who's around — Mom, Dad, the dog. I'm told it's a developmental thing; all kids do it at some point. But is it also a developmental thing that many, as they grow older, stop being so generous? Is it inevitable that we become protective of what we see as "mine," hostile toward anyone we see as a threat to our possessions, turf, ideology? Is Griffin just naive? Will life teach him otherwise? Or is he, perhaps, teaching us something — a gentle, smiling reminder of what it means to be fully human?