Fear or Aggressiveness??

[murphman]

Hi,
We are first time boxer owners and just love our 12 week old Murphy. He's very sweet, cuddly, and seems to have a good disposition. He's met a lot of people, both adults and children, at our daughters' soccer games. He does generally well in these situations and, if anything, is a bit too submissive. If people approach him, he will flop over onto his belly and also occasionally will have submissive urination. We try to tell people to squat down to his level and then he's much more comfortable.

Lately, he has started to bark at people walking by our house. I will have him on a leash in our front yard and he'll bark, some growling and the ridge of hair on his back goes up. I know at this age, he's probably not being protective but I'm guessing it is fear?? Luckily, I read all about the fear stages before we got him so we have been ignoring a lot of these behaviors. So my question is, at what point do you correct this instead of ignore? Also, can a dog be too submissive?