Imagine yourself back in the throes of puberty, and you're madly in love with someone who doesn't yet know how you feel...
You're at a park, enjoying an ice cream cone, when your would-be lover walks by, sees you, and smiles! You offer your beloved a bite of your ice cream, and you're OVERJOYED when the offer is accepted!
Back to the present... Now imagine you're eating some delicious strawberries. You only have two left, and you intend to savor both of them. Then your child sees the strawberries and says, "I want one!"
Are you as thrilled to share with your child as you would be in the first scenario? If not, why the difference? (Hint: love trumps scarcity.)
The way to raise kids who enjoy sharing is for you to enjoy sharing with them. Frequently offer them bites of your food, for example, and let them know how good you feel when you're sharing.
When your child doesn't feel like sharing something with a sibling or playmate, rather than forcing them to share (which isn't really sharing), find something that you can share joyfully.
As you consistently demonstrate your sharing ethic, they will eventually discover the joy of sharing for themselves.