Don't get me wrong, being generous is definitely one of the many virtues we extol and practice in our environment. But sharing? Not so much. At least not in the way most people think of the word.
We are surrounded daily by small humans learning the ropes of social interaction. Not so long ago, they were helpless, every moment of their little lives focused on having their needs met. As they grew, and more was added to their immediate surroundings, they began to focus on those things around them that caught their focus. Toys, food, the attention of the people around them, these things were added to a the child's wants/needs list. And I will state again what parents hear often, a child's wants and needs are the same as infants.
Then an interesting thing happens. They start to talk, and walk, and run. The world is widening. But that reality of needs being wants doesn't change. Toddlers need to be allowed to hyper focus, they need to be given undivided attention at times, the need to be talked to and listened to. They need to form ownership of part of their world. Everything is often still done for them. They exert very little control over their own lives, so small measures of control and ownership are cherished. Favorite toys may go everywhere with them, they may only wear a certain pair of shoes that they can manage by themselves, foods that are easy to manage by themselves become favorites.
At this auspicious time we adults do something awful. We interfere. We decide that other shoes are better, we try to feed our kids foods that are convenient for us, and we start to get it into our heads that our kids can't be "good kids" if they don't share. We've covered autonomy in daily routines and food, but sharing comes up with nearly every new family touring the school. "Do you teach them to share?". And we are often met with surprise when we say "Absolutely not!"
First off, what would you do if someone came to you and asked to use the pen, computer, restaurant table, you are using? Would you just give it to them? That's the sharing expectation we have for kids. And it's silly. How about if your friend came over for coffee and went into your room to try on your shoes? Or maybe saw that you had a new car and decided that they wanted a turn. We are honestly expecting more of our kids than we do of ourselves. And it's confusing for them too. Often times new children come crying to us because another child "won't share". What they are expecting is for this other child to simply hand over whatever they have because the new child wants it. How else would they see it if their own experience has been that friends can come and take their things without their true consent under the guise of sharing?
This is what we do instead. We teach compassion, and fairness, and assertiveness. We teach the children that they can ask for a turn, but the answer may be no. We teach that if someone asks for a turn, you may negotiate. We teach that other options may be available. We teach that just as you may not touch someone else's work, no one will touch yours. We teach that screaming and demanding are NOT how we get what we need. We talk to each other, and adults will help you, but they will not do it for you.
In short we teach children how to get what they want and need without subverting what others want and need. Ultimately sharing means finding a way that is fair for everyone.