You guys remember how Rocky was possesive with the rawhide? Well, lately he's been possisive with toys and food also. He won't let our other dog eat because he thinks all the food is his. He growls if you approach him when he has a toy. He's also been finding ways to get attention such as by urinating inside, stealing stuff (socks, shoes, gloves, hats, etc.), and tearing apart papers. He's never been like this before and not only does he snap at me but he just snapped at my brother a few minutes ago because my brother took a napkin from him. He has never shown ANY signs of aggression before, not even growling. And he's always been a bit of an attention-seeker but not this bad. All this behaviour has started after getting a second dog...I'm beginning to think getting him a companion was a mistake

. Please help!
Note: He's a 1.5 year-old male nutered mini-dachshund.