Overly concerned

Lately Michael has been overly concerned with other people's things. For example, if we get in the car and he sees Rory's blanket, she better have her cup too or he gets upset. The other day he almost started crying because he was so upset that she didn't have a cup. Rory couldn't care less about not having a cup but he couldn't get over it. Then tonight while we had some dinner at Costco, he saw another little girl with a cup that looked like mine (since it was just a regular paper cup from the food court). He was really upset for a minute and almost started crying again. I asked him what was wrong and he said "That's mama's cup!!!" I had to show him that I in fact still had my cup before he would settle down. Good thing I didn't toss out my cup before that or he probably would have gone into hysterics. To him, once an object has an owner, nobody else is supposed to touch it. It's difficult to tell/show him that it's okay if I want to share my stuff with other people. Things have to be set in their perfect ways and I guess by sharing that throws too much unknown into the mix and he doesn't like it!