It takes a long while for most children to be able to share. For one thing, children do not understand what sharing means. If you ask them to share something, they think you are taking it away from them. They do not know they will get it back. Sharing will come in time, as children live and grow up with adults who share.
Great good can come from sharing.
We adults can help children come to know that. We can also help them with times when sharing is hard. Let your child have some things to own. That is why it helps your child to have things that belong to just him or her. Let your child have certain things that others do not take without asking.