Samoyed dog unfriendly with strangers...

Hello everyone,
I'm new in this forum. Though I've pointed out some problems with my Samoyed boy- Sam, I'm just specifying it here.

Sam is very affectionate, playful and obedient. He is cool with his buddy Onyx- my Norwegian forest cat. He is also usually friendly towards other dogs and animals. He recognizes all my relative perfectly even meeting after an year or more. He is potty trained and neutered- not noisy at all. :D

Sams problem is - he is unfriendly with strangers. If I'm not around he'll try to scare him away by aggressive barking. If he's challenged (by eye contact/ harsh words) he'll chase him/her away. But he doesn't bite that time though he is too fast to catch up. Secondly, he is too adorable to resist touching and this caused several people get bitten. As I said he's too fast to use his fangs that he can bite 3 times per second, he also uses his claws- he can make similar damage. He never bites a stranger unless s/he touches him.:confused:

It will be very helpful if you guys give me some suggestions to make him giving up these issues asap. :(

Thanks.
~Tanvi
(PS: He's usually more aggressive when he is leashed. Surprisingly he can easily tear a steel chain or untie a nylon leash if he wishes.)