Consider Their Age
A baby or toddler isn't capable of true sharing. They don't have the language skills or the grasp of time necessary to understand that they'll get the toy back later. Two-year-olds are egocentric by nature (that's why they sound like the seagulls from Finding Nemo), and "give and take" just doesn't jibe for them.

A preschooler, however, is starting to develop the capacity for empathy, which makes sharing possible. That doesn't mean it's easy for them. They'll need to be eased into it with lots of gentle coaching. By school age, you might witness a deep sense of equity or even benevolence, but remember, they're still kids.