When he rushes at you, barking, are his ears up, pointed forward, or laid back? Is his tail wagging or down? Those are the clues to see if he's excited or anxious. Shelties tend to bark in either case, so the rest of his body language will be a better indicator!

I knew a beautiful Standard Poodle who was terrified of anyone blowing in her face, after a not-dog-knowledgeable co-worker used an air hose pointed at the pup's nose when she was just 9 weeks old.