sounds like my son

but my son probably has Asperger--he has trouble interacting with other kids normally. he can't read other people's body language. he just takes a few cars and play by himself on the side. if i take cars away from him, he throws a big tantrum.

i try to divert his attention from cars, and reminds him to play with other kids. before the end of the playdate, i will start telling him to put away cars, and then i will take him to play with other kids. i don't allow him to play with cars by himself for longer than 10 minutes at a time, because if he plays with cars for too long, he will have a hard time getting out of his "car mindset". he is much better now. he understands that he cannot always play cars by himself.

today, he was playing with someone's cars by himself, and i said, "i will count to 5, and then we leave the cars. 1-2-3-4-5." i got his attention. he put down the cars, and immediately i said, let's go to find holly and heidi (our new neighbors that he liked). then i picked him up and started to run away from the cars. he couldn't help laughing, and forgot about the cars for a moment. soon we left the friend's house, and went home without a tantrum. just a few weeks ago, every time we left that friend's house, he would scream and cry and have a tantrum. today he did just fine.

for you, you need to avoid/prevent the bad situation, rather than try to fix it after it happens.