Dogs are born with levels of drives: prey drive, food drive, play drive, social drive, and even fight drive. Typically dogs that exhibit this behavior, have high social and play drives, so in human terms that would be a “physically active extravert”. What happens is these dogs are out walking on a leash with their owner, and they see another dog walking down the street or at the park. The dog will typically pull forward, cry, moan, and exert many signals that they want to go interact. Typically, if the owner is out for a walk, they keep walking and ignores the dog’s request for social interaction. Over time this unanswered request (from a dog that desires it badly) becomes very frustrated, and when a dog becomes frustrated they typically start barking and growling. They also associate this frustration with the leash. Over time this can become habit without even thinking. When they are off leash, that same frustration has not manifested because typically owners have dogs off leash when they are allowed to play. (Please understand that the answer to this problem is not to let your high social drive dog run up to every dog he sees because that can be dangerous. Allowing your dog to meet his social needs in a safe way while providing the proper structure will resolve this). Other scenarios that can cause this behavior is a dog that has been attacked on leash, so they (out of fear) expect a fight, so they go on the offensive to push dogs along verses waiting to see if they are going to invade his space.