Can a dog trainer help us with our dog who has become aggressive with other dogs that pass by our house? He's 8 and this behavior started a year ago. He used to lay quietly and let them walk by. We've been super careful and able to keep him confined to our back yard but today he went after a neighbors little dog completely unprovoked. Thankfully he didn't hurt her, but obviously this behavior can't continue. Vet says he's healthy. Thoughts and suggestions? We have an electric fence that he will not breech anywhere but the front when he sees a dog.