Leashes Can Cause Aggressive Behavior
When dogs are off leash and encounter other dogs or stimuli, they are free to get away, display a “don't bother me” attitude, or simply invite the other dog to play. Dogs on leash cannot show these same emotions, and they feel more cornered and threatened.
Leashes and the owners who hold them can make it impossible for dogs to give the appropriate signals to each other and tend to get in the way of communicating rather than helping the dogs get along. Dog owners who fear their dog's reaction to other dogs often don't help matters because they tense up and tighten the leash, signaling to the dog that trouble lies ahead.
People with on-leash dogs tend to approach each other head-on, whereas dogs normally approach one another in a curved half circle. Approaching head-to-head is a combative signal that says you mean business and may want to fight. Your dog may recognize another dog's rank and lower himself ever so slightly to signal to the other dog that he means no harm.
As a human, however, you pull yourself and the leash upright when you see the other dog coming. Pulling up on the leash changes your dog's body posture into a more threatening stance, causing the other dog to react and your dog to become defensive. The higher-ranking dog is taken by surprise and attacks your dog for changing his mind and posturing the wrong message at the last moment. No wonder it is difficult for on-leash dogs to get along!