Sharing is not instinctive
The fact is that children are not born with an innate sharing instinct. Very young children have not developed the ability to view the world and the things in it from anyone else's point of view other than their own. There is no specific age when children magically become unselfish and willing to share. Sharing is something that develops with maturity and every child matures at his or her own pace. However, the right environmental influences can give them a nudge in the right direction. Children learn to share more quickly in a household where adults give and take and exhibit cooperative behavior Some children take longer to share than others because they have the kind of temperament that is resistant to change.
Tips on teaching your child to share.
Do not make your child feel like she has a character flaw by labeling her as selfish and warning her that her behavior will result in her having no friends. Accept the fact that there are some things that your child will just not be willing to share. In such a case, when you invite other children over to play, help her identify those toys that fall into the 'hands off' category and put them away before the other children arrive. In this way, you can prevent skirmishes over toys.
Your child must be aware that just as she is possessive about her things, she must learn to respect other's possessions too.
Lay down some ground rules at the start. For instance, tell her that she will only be allowed to invite other children over if she shares her toys. Or, you can tell her that she will have to share the new video game with her brother or forfeit playing with it at all.
Create situations at home where your child will have no alternative but to share. For instance, if another child has come over give them one set of crayons between the two of them. Show your child that you can share too. If your child is having a hard time sharing, you may need to supervise her playtime and encourage her to share. Praise your child every time she shares or makes an unselfish gesture.