[QuickSilver]

Dogs have an interesting way of viewing "resources". Humans are definitely resources. When I helped supervise play sessions at my local shelter, I was told not to lean against a wall, because that made me seem more "resource-y", and thus a temptation to guard.

Dogs tend to view resources in terms of proximity. If they are next to something of value, it belongs to them. Many dogs will decide that they own the human whose lap they are sitting in, even if they've never met that human before.

Thor has an interesting take on this. If we are sitting and someone approaches us, he barks his head off, no matter who it is. However, if that same person is sitting in that spot and we approach THEM, Thor is all sugar and spice. To him, the act of approach is much more important than the players or the location.

I agree with others, start the behavior mod immediately. Teach him that guarding results in loss of lap time, or a time out in the bathroom.