My dog is obsessed? why?

Ok so i am 15 and I have an english mastiff he I three years old loves all his toys and food. We recently got him fixed, and he has this huge rawhide bone which may be becoming an issue. Everytime he has it out no one can take if except me if another one of our pets comes next to him he growls and snarls. He was never like this but he seems to be obsessed, he will chew it all day if you let him. Anyway I wanted to know what is making him so agressive over one bone, why is he obsessed?