Cosmo is almost 5 months and showing a few signs of aggression, when we go to take off his leash he gets snappy and when we punish him, is this normal for such a young age? It just started about a week ago but I'd really like to get it under control as soon as possible. I am constantly  rewarding him for his good behavior and just saying "NO" when he gets snappy. Any other ideas? 

He is such a happy puppy so I dont know where this behavior is coming from?! 

Please…