At a out door play day with my ds, 3.10 years, I had a change of thinking about sharing.
We have went to play groups since he was really young, and I've always encouraged sharing. Give the boy/girl the toy, share!! And 8/10 times he has without major problem.
Today he was playing in the mud kitchen, making tea with a tea pot, having great fun, an older girl comes along asks for he Tea pot, my ds says no, I say share, he says but I'm not finished playing yet mummy, and upset hands it over. And ends up wanting it back.
Would it have been so bad to say OK to my ds OK, and say to the girl, he's not finished playing yet, you can get the a shot in a wee bit?? "