I'm kind of surprised how many people have the "not my dog attitude". "He/she listens, I trust them". When the reality is quite different. Just because your dog is good around people doesn't make them a good dog. How do they react around male dogs, or female dogs, or puppies? What about when people are on bicycles, or rollerblades? Despite my local beach having a "all dogs must be leashed" sign, my dog and I have been charged by unleashed dogs quite often. My dog is still a puppy and being only 5 months old, bigger dogs have knocked her around a bit. I've also had to pick her up because of aggressive dogs, looking to assert dominance. Bottom line is, if the dog is overly aggressive, I'm going to step in, and it won't always be in a friendly way. If you don't have a leash on your dog when you're supposed to, well, if something happens, you're going to deal with the consequences. Although its not your dogs fault, they will be the ones that will suffer for your stupidity.