Dogs tend to have a greater need to go visit with or bark at a dog when they’re on leash for a couple of reasons. Some dogs have a tendency to become protective of their owner on leash and will react towards other dogs that come near. What the dog is telling us is that they don’t trust the owner with the situation they’ve been put in, therefore they must make the decision to ward off the stranger. With training, the owner can establish a level of communication with the dog, showing them that they do have control of the situation. By asking the dog to sit/down or simply telling them “No” for unwanted behavior, the dog will learn to respect the owner and trust them in new environments. With a dog who is shy or nervous around other dogs, some owners will also experience an elevate reaction from their dog when they are on leash. A simple analogy that may explain this reaction could be the situation of a group of kids coming to bully you, but you’re tied to a tree, with no escape. You might come out swinging. It’s as the saying goes, “Fight or Flight.” If you can’t get away, much like your nervous dog on leash, then they’re going to stand their ground. For your dog, this may produce a level of excitement/aggression that your dog, in an off leash setting, wouldn’t typically do. Again, a group obedience class could help your dog relax in similar situations and learn to trust other dogs, but especially you for putting them in that environment.