Grrrrrr this dog!!!! :-(

We have decided to get possesive. Very posessive. He just took the usb bluetooth thingy and decided to chew it up and when we tried to get it back it was a ginormus wrinkled nose, snarl, growl and snap

I'm very upset with him. He will let you take his food off him anything. Normally if it's something he has stole he will make it into a game of chase or most of the time just give it you. Not lets try bite whoever comes near me to take it off me :-(
He dropped it after about the 5th treat we showed him, but when my mom picked it up he launched himself at her, i managed to grab him though. Now he is sitting looking sad as i think he knows he has been bad (With the 'bad dog' shout i gave him when he did it i think the whole world knows!!)
Anyway why is he doing this? :-(

It's scary to think if he had something very dangerous in his mouth that i couldn't get it off him :-(
Thanks in advance, sorry this was a bit long.
Beth x x x x x