I've noticed that even the smallest toddlers, whether they're digging around in the sand at the playground or visiting a friend's house, get the occasional prompt to share and wait their turn.
It's something most parents expect from their kids starting from a young age. The problem is, until they are three or four, children's minds are not yet set up for cooperative play – their brains simply don't have the necessary building blocks. Turn taking and sharing are not just polite social behaviours, they involve highly-advanced skills like impulse control ( resisting the temptation to grab the toy or tackle the eager onlooker hovering next to you). Keeping impulses at bay is an ability that children develop slowly over time – each at his or her own pace. Even teenagers haven't fully mastered this ability yet (or else they would think twice before sneaking out the back window or texting until dawn), so toddlers are far from being experts.
The other major hurdle to sharing is seeing the world through another person's perspective. What's the point of giving up a much-coveted shovel and pail, when you don't empathize with the other person's feelings? Kids make a big leap in empathy at age four, when they first start to really grasp that the world doesn't start and stop with them, but again, this process is a work in progress for many years.
In the first year of life, babies rarely even notice when a toy is snatched. At this stage of development, let it go and resist the impulse to make things "right" by exchanging toys again. When they do start to care, if you're the parent of the snatcher, get down on the child's level and say, "I saw that Emma was playing with that and you took it. She looks upset now." If possible, try not to immediately take the toy yourself, but let the words sink in first. As the parent of the snatchee, encourage your child to use words like, "I was playing with that," instead of yelling or hitting. If you narrate the interaction for your child first, it may still be messy, but it helps them understand the social dynamics of how to navigate it.
It's important to introduce the concept of sharing to young kids, but try not to get too frustrated when they struggle with it. Remember it's a sophisticated skill that they will spend years working on – in the meantime, all that crying, grabbing, and hoarding is like a testing ground for their little minds.