Hi I have a 6 month old lab. He LOVES everyone and every dog, until recently. He now has this odd aggression for dogs that look like Pit Bulls and one or two other stocky dogs in our complex. He plays in the dog park for hours on end, no problem but if he sees these dogs while on the leash he makes these horrible growling noises and lunges. The funny thing is though, his tail is wagging like he happy excited. I don't get it but he's a strong puppy and he's scaring my neighbors. How can I change this behavior? I'm so frustrated.