Because:
#1) It's not natural for the dog. We're not talking about human children who need to be socialized with other kids throughout their infancy. Dogs learn dominant and submissive behavior and how to interact with other dogs from 6 to 8 weeks of age. This two week period is called a critical stage, and a small amount of exposure will have a lasting effect on your dog's personality.
When you throw your dog in with all kinds of other dogs (from other packs) the first thing they need to do is establish who's dominant and who's submissive. And yes, they'll tussle to do this, often. If you have two really dominant dogs, they may even fight to the death. Or if another dog gets flushed too quickly, he'll get defensive. And then you have a dog fight on your hands, with hundreds of dogs and owners yelling and running around screaming.... and none of the dogs are trained... and none of the dogs are on leash... and all of the owners don't know anything about dog handling (esp. a fight) but think they know everything. Trust me... it's a bad situation you need to avoid.
#2) Health: They let anyone into those dog parks. And believe you me, you get the types who will find a dog in an alley and before giving it shots (rabies, parvo, etc..) ... they think they're doing a great thing by bringing the dog to the dog park where he can cough, lick and breathe on your dog.
#3) Temperament: Nobody does a temperament test on these dogs before letting them into the park. Duh! You're playing with fire.
So you can see, there are a lot of risks. And just because the dog gets into a dominance scuffle, does not mean that he's a dog fighter. But that's a different issue for another article.
That's all for now, folks!
Adam
