Many a time have I been astonished at how possesive parents are of their children's toys. They don't refrain from yelling at other people's kids because they might break it. They would rather pull something out of a baby's hands than let him take it home to return it later. They yell at their own kids when they break a toy.
If toys have such power over parents, why then should they be so confused when their children won't share them? This possessive behavior - over any object for that matter - only teaches the children to 'defend' their objects, if not for their own gain, then for the love of their parents, because if the parents value it so dearly, it must be worth fighting for. Surely mama wil be happy if she sees how I guard my property. How confused must that child be when the parent then screams 'stop! Share your toy!'.
If a child hears nothing during the day but possesive nouns, don't be surprised if he in turn turns out possessive.
Moreover, if you call your child's toys his, then they are also his to break and his to mourn over. It relieves you from the right to get angry about breakage.
No object is worth the tears of a child. Toys are merely replacable things - objects of diversion. They needen't cost much to entertain your child, and if they did - and end up broken - you only have yourself to blame to have fallen in the trap of commerciallism.