My 4 month old pup is starting to become a little defensive around his food. Not so much to me, but other dogs. He has growled at me twice when I tried to take something he liked chewing on away, but I responded by taking it and telling him no, showing him I would not tolerate that and that I control the food. But the other day my friend brought her dog over, and he got aggressive each time she (her dog) went near his empty food bowl. Another time this happened was at the park around some treats that weren't even his. I've responded each time by telling him no and removing him from the situation for a minutes or two. Should I be concerned? What can I do to prevent more of this?