It's one of those things many people consider a staple in raising 'good kids'. Kids who're sweet, nice and who share their toys with others, no questions asked. It sounds great in theory, but it's not always realistic, especially when it comes to translating into adolescence and adulthood.
I always figured it was easy and expected until I had my daughter. I would take her to pre-school or the park and she'd be happily playing with something. Suddenly another kid would come and grab it, and I was expected to tell my 2 year old to 'share'. Share because another kid couldn't wait their turn?
Because another kid saw my little one playing and wanted some of that, too? Because I didn't want the kid to cry and embarrass me in front of the other his/her mother?
I honestly don't know exactly what it was, but it just seemed like 'good manners' to teach her to share... even though it sucked. It sucked because my girl is very sweet and gentle (no biased, just honest), and it seemed wrong to validate the behaviour of a kid who'd come over and snatch a toy. It never sat right with me. In saying that, I kept doing it because it's just 'what you do'. Recently, however, a mother with an online presence has come out and explained why she doesn't force her son to share toys. It's a seriously great read.