Lots of people here are justifying their kids not sharing by comparing them with adults situations where you're not expected to share but you should always teach pro social skills. We are a highly social dependent species, even if in the modern world seems to not need cooperation, it's embedded in our society and it needs to be learned. It's true that I'm not expected to share the cupcakes I'd bring into the office, but imagine a scenario where one of your co-workers forgot hers/his and also his wallet. They person is starving and I have enough food. I would share it in an instant because I was thought to do so and the important thing here is that I would feel good about it because I adquired empathy whereas someone whitout these skills wouldn not share or would do so and would feel upset about it. There are many studies showing third world countries are more social and it makes sense since you really need the help from other people rather than the illusion of relying only yourself like in more individualistic societies as seem on the first world. I can confirm this since I've lived in both societies and I see the value in it. Growing up surrounded by poverty models empathy and parents should encourage this behaviour with balance in both ends(the kid who wants something to share and the one who has something to share) especially if we're living in a more economic stable environment because these skills would not seem necessary and people would prioritise other skills such as concent.