hey everyone, i'm going through something very upsetting with my viszla, bode, and i dont know what to do. this started about a month ago...he is 15 months, so lets say this behavior didnt happen for his first 14 months. when he is on leash, and only then he starts to aggressively growl and bark at other dogs his size. he does not do it to little dogs or dogs he knows. it's very embarrassing because he's really strong and i actually have trouble holding him back and pulling him away. i refuse to be the person who says 'my dog's not friendly'. we go to the park every day and he hikes and plays off leash for over an hour. this never happens at the park when he's off leash. he has to be on leash when i take him out where i live. any suggestions?