Yes, I said it and have been saying it for years.  We all find ourselves struggling when we are teaching our child(ren) to share.  That is because they may think the action behind sharing means that something is being taken away from them.  So I will be the first to tell you to forget the word sharing for a while and replace it with the words "take turns". 

Do you remember the game, the ball goes round and round?  Well kids wanted to play because they wanted to get the ball back and forth, but didn't want to be the last one to have it.  Hmmmm!  Lets work with that...Do you think that kids will want to continue to play with a toy if they know they are getting the toy back after they have given it away?  The answer is YES...therefore taking turns is the way to go. 

When children are young, you can first explain the rules for taking turns.  For example, help by encouraging them to sing a song or just count to ten and then switch hands.  This helps them understand that after they are done signing, counting (aka being patient), they will be rewarded and get the toy back so that they can have a turn.  It is a great lesson for all and a complete shift from I dont want to share, to I will gladly take turns.  Same behavior just different language and approach.  So stop sharing and start taking turns and you will struggle less with this very common social development skill and enjoy watching them have fun together with one toy...that is until they get sick of it (within one minute usually)  :0)