There has recently been an interesting dialogue among moms I know about the concept of sharing. Extrapolating from my experience with Montessori, I wanted to attempt to give the thoughts I have formed on the subject:
-Learning to wait is good. In a Montessori classroom, there is generally only one of each material or exercise. Whether or not this is an intended purpose, having only one of each material ensures that children to develop an understanding of what to do when something they want is unavailable. The classroom has its own protocol for communicating availability (when an item is on the shelf, it is available). Learning to delay gratification is valuable, and there are a variety of strategies that can be taught when this opportunity arises (use another material for a while, count to 20, ask politely when the other child will be finished, write your name on the board in a particular spot...). In fact, I recently read a study popularly known as the "Marshmallow Test" that indicated that 4-year-olds who were able to delay gratification at age 4 fared significantly better in high school - academically, socially, and emotionally.
-Concentration is worth protecting. If a child is working intently with a material, forcing him to share it may disrupt whatever he's doing. I definitely believe that a child who is continually disrupted learns not to engage in deep concentration, just as we would become insomniacs if we were continually awoken moments after falling asleep. Allowing the full cycle of activity - engagement, concentration, and disengagement - has tremendous benefits for a child's sense of satisfaction and accomplishment.
-A child will only learn to share when it is truly sharing. The above statements assume a model of sharing where another child's will or the adult's will is imposed on the child with the envied material. In either case, "sharing" on the part of the child is not the act of generosity we generally associate with the term. I genuinely believe (and I think Montessori would agree - she discusses this in relation to adolescents and society) that virtue only comes from experience of its benefits and of the negative feelings associated with vice. When a child chooses to share something, he can own the feelings of happiness he's given to another. And he may, on occasion, need to feel what it is like to reject someone's request for purely selfish reasons. I think we as adults can use these opportunities to make salient the emotions and needs of others, but we need to let children choose, as often as possible, what they wish to do with what they have.
I say this all with the caveat that my thinking only applies to very young children. I haven't yet sorted out my thoughts on children within the second or third plane of development, where rules, negotiation, and social custom play a larger role. I'd love to hear others' thoughts on the matter, or relevant experiences that may help to develop my thinking on this complicated subject or bring me more in line with a Montessori perspective!