I have a four year old German Shepherd mix, and he's generally really friendly towards other adult dogs. However, whether he's walking on a leash or at the dog park, he always barks at puppies, snarls at them, and sometimes knocks them down so he can stand over them. I think he has a hard time handling their energy, but doesn't seem to respond in an appropriate way. I'm not sure how to correct this behavior. Does anyone have any thoughts? Thanks!