How many times have you asked your child to share toys or your attention? Most likely, a lot. The concept of "share" for children at this age do not exist. The child strongly believes that he is the center of the universe, and that he and only he deserves all the toys (including foreign) and all the attention. Your baby has an egocentric view of the world at this stage of development.
He is convinced that everything they think and feel exactly like he. And he can do quite rude to other children, because your child is not aware of the feelings of others. If, for example, another kid tries to take his toy, he can grab his dramatically, or worse to push or hit the "enemy."
Of course, you want to wean your baby to fight with his friends on the game, but do not expect that he will share toys, or at least understand why it should be done. The principle of "my" taking away other people's toys and defend their property (to the last drop of blood!) for eighteen months the baby is not nothing but another way to assert their autonomy, to know their limits and learn to defend their interests.