We'll have had our lurcher Dylan three years next month (how time flies!) When we first got him, he was very excitable and noisy when near other dogs, and the behaviourist we saw diagnosed fear aggression. She said he was confused..he wanted to play, but when dogs got too near, he was afraid they would go for him, so he started acting up to scare them away. With a lot of patience, he is now much much better, he practically ignores them unless they growl or bark at him...until we started walking our puppy cav with him. Now if another dog comes any where near, he goes absolutely berserk, growling and lunging. Rosie meantime is happily wagging her tail at the stranger and wanting to say hello!
I would imagine he thinks he must protect her...even from dogs which are putting out their best friendly signals. Short of never walking them together, which isn't really practical, how do we convince him he doesn't have to do this? I don't want him making her think that other dogs are to be shouted at/attacked/feared, as at the moment she has the usual "I love the world and everything in it" cavalier temperament.