Perhaps I'll catch some flak for this, but I encourage protection and a little aggression in my dogs. Thats one of the reasons I always have a Boxer - for added security. It's the all natural answer to house alarms, and has been since man and dog first hooked up thousands of years ago.
My female Sheba (guarding the bridge), was in a word, tough. She only weighed 45lbs of sold muscle full grown, but boy was she tough. Among the many instances of protectiveness I can recall was the time I went to put her out in the yard and we both discovered a Black Lab male in the yard. I told him to "git" and I think Shebie, with her large vocabulary, misunderstood that I was telling her to get him. She immeadiately broke free and pounced. It was the most amazingly suprising sight - she jumped on his back, and bit the back of his neck and shook it. Mind you she was not tall, so though shocked, I was still amused to see all four of her legs dangling over this large dogs ribs, "going to town" on his neck. He got the picture quick and managed to extricate himself from her grip, running out of the yard as fast as he could. Of course I had to chastise her, but secretly I was pleased to see such bravery, spunk, and fiestiness :D
Another time, some guy down the street was walking to the yard looking quite menacing as our kids has just had an altercation and I suppose he mistakenly thought he was going to intimidate someone. Again, Sheba's response was immediate and quite bold. Though she was on the chain she still ran full throttle toward him, hair on back raised, barking ferociously. He had the nerve to pick up a brick in MY yard and I suppose was contemplating throwing it at MY dog, in MY yard. I took care of that one *wink.* And yes, Sheba was the most loveable, friendly, fun loving dog. But, she was ferociously protective when she needed to be and I believe that is an acceptable balance.
Now, about my Bing. Though barely six months, he is exhibiting the same behavior, luckily. My bedroom window is over the front porch where I have foliage growing on a small track of land. In the midst of this foliage was a small statue with a glass ball. One night at about 5am, Bing woke me up barking very loudly. It was then that I heard a male voice say "Good boy!" So, after a minute of letting this register, I jumped out of bed and ran down the stairs, out the front door to discover that my glass ball was gone, and a guy was sprinting down the street with it (the nerve, stole my ball then told my dog "good boy" for barking). Had I been on the ball, I would still HAVE my ball. Bing, on the other hand, was right on the mark.
I like a dog who takes his job seriously, and knows by instinct when someone is cool and not cool and responds appropriately.
The key is encouraging them to be their friendly, fun loving selves, but also praising them when they've done well with their other responsability - protection.
BingsDad