When Lily was a puppy, I was anxious to socialize her with every dog we met. I knew with her breed, American Staffie Mix, that we could do all the work for the first two years and she still might become dog choosey when she hit maturity. What I hadn't counted on was how little owners actually know THEIR dogs. Before Lily was six months old, she'd been bit more than half a dozen times by neighborhood dogs. The little ones. Lily is incredibly tolerant. I am not. After the first two times, I realized "my dog is friendly" meant "I think my dog is friendly but I have no clue". I stopped letting her greet on leash for her own safety. She still got bit. It turns out people with little dogs in our neighborhood not only don't train or socialize, they also use flexie leashes but have no clue how to control those. Lily would be in a sit-stay ignoring them when they would rush up and bite. She has scars.
This is a perfect example of why I don't let other dogs greet her (for her own safety) - Great Dane attacks English Bulldog. Dog-dog aggression isn't limited to a few breeds. It can happen in any breed - any dog, any size. Not every owner knows how to read their own dog's signals. Not every dog that is friendly off leash in the dog park enjoys being greeted in the park when they are on leash. Some owners learn the hard way - at the expense of their dog. I'm not willing to let Lily pay the price for someone else's lack of knowledge.
As we were sitting outside Starbucks last weekend, Lily was lying at our feet. A Chocolate Lab was pulling his owner toward us. The dog lunged at Lily with his teeth bared, snarling. Lily didn't react, choosing instead (thank you, training) to look to us for a treat and reinforcement. The dog's owner chirped, "Oh, I know, Brownie. You just want to say hi." I can assure you, that dog had no desire to greet, but the owner didn't realize that.
I had a trainer point out to me that it's not normal for dogs to greet every dog they see, and she finds it weird that owners feel the need for them to. Do you greet every single person you pass on the street? I certainly don't. Our dogs shouldn't either.
Socialization is important. Socialization with well behaved, dog social dogs. This can be done through playdates instead of on our street, with our dogs off leash instead of at the end of one. It's better for all involved.