I'm with brdgirl and sunnydelight, I'm generally pro-sharing as an way I;d like to see society run.
For an argument in favour of sharing, at a big, species/society level, you can do worse than to read up on "The Prisoner's Dilemma" and, if possible, for people who have extrapolated that out into how societies and species might operate.
A problem, of course, arises when you realise that, actually, teaching your children to "share" might also be teaching them to "lose individually for the good of the species/society as a whole". Basically, if you have a society of sharers, and you are one of a small number of grabbers, you will do very well as a grabber. And, as adults, I'm sure it hasn't escaped our attention that this is the situation that would appear to prevail in RL. "We" pay our taxes, contribute to the public good by keeping within the law, picking up our litter, helping one another out, sharing abstract things and practical things (within reason). But it would appear that there really are those who amass vast sums of wealth by doing really rather dodgy things, taking advantage of tax loopholes, squeezing the juice out of the public goods, and not sharing what they've grabbed.
It must cross our minds occasionally that it might be in our children's best interests to teach them the (individual) usefuleness of grabbing - or maybe that should be "strategic grabbing" - grabbing occasionally, so that the system of sharing continues to operate (which of course makes stragetgic grabbing effective and possible). We don't want the system of sharing to collapse - what a nightmare - but occasional grabbing is really very effective.
As it is in the playground, the playroom, with toys, so, it seems for society.
By the way, all the above was explained to me by my child. I used to, and still do, insist on sharing, on helping others. And I will go on doing that. But, alas, my ds is a "wide reader", and I do remember that, during one discussion on the value of sharing, he interjected with the concept of strategic grabbing ....
... the joys of parenting.
for good or ill, he does continue to share, by and large. He's a teen now.