When people talk about sharing and toddlers/children it never ends well. There are some who believe that forcing your child to share isn't really sharing at all but glorified snatching and there are others who think you should encourage children to share. I'm going to get splinters here and announce I'm on the fence. I do think you should encourage taking turns, which sometimes can fall under sharing. However, we have always encouraged Nate to share by sharing with him. Sounds strange but we share most things with him, if we are eating we don't make an issue of him having some off our plate but welcome it. On the whole he is good with sharing things, he has started to call things "mine" but isn't as possessive as some children with "mine" objects. They are simply his, but mummy, daddy or Joe Bloggs can play with them but remember they are his.
Recently we have started to realise he is insistent that everyone has something. Nate wanted a biscuit so we offered him one and he picked three up (one at a time) we said "only one" his reply was: one for him, one for daddy and one for mummy. Very sweet. Even sweeter he'd finished his biscuit and wanted another so I offered him my one. He started to cry because he didn't want me to not have a biscuit. I love it so much that Nate likes to share with us.