When my kids were little, we had the motto that "We share our toys" and it was enforced. They had to share. At the time, I thought I was teaching them to be loving, kind and unselfish but looking back, I see that there are problems with that. Forcing sharing teaches by example that people's things are not their own and that we are entitled to have what someone else has or they are being "selfish." This is not what we want to teach them.
A much better way to teach sharing is by showing them the opposite example! When a friend shares because they WANT to share, that is a great feeling for the giver and the taker! Those good feelings make the child want to share more-for the right reason-to make someone else happy- and not because they have no choice over their own things. Think about it-do you HAVE to say yes to any friend who asks to borrow your car? Of course not-and it doesn't make you selfish.
I see this in action with my grandchildren. If Calvin has a toy that Stella wants, he is not forced to share and vice versa. It is simply stated that "Calvin doesn't want to share right now. Here is another toy you can play with." He often does want to share and it is a sweet moment rather than a forced "you have to share." It is important to teach kindness and sharing by good example-for the right reasons and not just because "that's the rule." We need to teach good actions WITH the right motives.