Even friendly dogs should be kept on leash

Me and my on-leash dog enjoying a day at the park

The other day, I pulled into my favorite park and spotted a man sitting at a picnic table with his two dogs. One was on leash, the other was not.
I pulled the Jeep near him and rolled down my window.
"Excuse me, sir," I said to get his attention. "I noticed you have a dog off-leash and I'm going to be walking my dog. He's not friendly and I was hoping you could put your dog on leash."
Let's note that I totally went the polite route with this request. I could have scolded him for breaking park rules that require all dogs to be kept on a six-foot leash at all times, but I've always figured you get more bees with honey, right?
"It's OK, she's friendly," he said back to me.
"Well no, it's not OK," I said. "My dog isn't friendly, and if she approaches him, he'll bite her."
Sometimes you have to spell it out to get people to understand, and he did, and put her on a leash. Sensi and I went off on our walk without a hitch.
Unfortunately, this is situation that I hear about a lot from folks, even folks with pretty good, even-tempered dogs.
Even my husband and I debate this.
"Don't you think, if we had a perfectly friendly dog, that we wouldn't see things this way? That we'd want to let him off leash?" he asks me.
My reply is always the same: "No, because even when we do have a perfectly friendly dog, and one day we will, we will remember Sensi. We will remember that if that wonderfully friendly dog wanders up to a not-friendly on-leash dog, it will probably get into a fight. We will remember."

How dog behavior factors into on-leash aggression
OK, so let's start with some dog behavior facts.
One — A dog that charges head-on toward another dog is making a threat. The charging dog may not intend a threat; perhaps it is charging because it is over-excited and has poor doggie social manners. That is often the case, I think. However, the dog on the receiving end of the charge is likely to perceive the behavior as a threat. The only time that this may be entirely non-threatening and understood as such by both parties is amongst dogs who are good friends and play together frequently.
Two — Proper dog behavior protocol for introducing ones' dogself to another dog includes keeping a comfortable social distance and avoiding prolonged eye contact until proper signals have been exchanged. The signal exchange comes after dogs have had time to scent one another from a comfortable distance apart. After an initial whiff from a safe distance, dogs will give tons of tiny little signals that communicate whether it is desirable to decrease social distance. It is only after these things have been undertaken and both dogs have agreed to decrease social distance that butt-sniffing and rompin' good play takes place.
Three — Many dogs become overly excited upon meeting other dogs. This does not mean they are bad, aggressive dogs. It usually means they do not get to see other dogs very often or did not have a lot of doggie social contact in those formative puppy months when dogs learn how to properly introduce one's dogself to another. The danger of an overly-excited dog is that it is an unstable energy, it is not happy nor confident nor friendly. It is just excitement. This unstable state of mind can quickly devolve into aggression at the drop of a pin.
Four — Fight or flight. We are all familiar with this, right? In situations where an animal (even us) feels threatened, it will opt to either fight or flight its way out of the situation. When restrained on a leash, the dog effectively has no option to employ flight. This means if the situation suddenly turns from extreme excitement to slightly unsure or a little anxious, the dog will likely turn to aggressive signals to communicate to the other dog that it wants to increase social distance (get further away). Dogs in close physical contact who employ aggressive signals are much more likely to bite or be bitten, as these communications, and the behaviors they warn of, generally happen in mere milliseconds.

Analyzing the on-leash, off-leash meet that turns into aggression
So, let's say I'm walking a pretty friendly dog. This dog has never shown aggression to other dogs. It lives in an only-dog household, so the only time it gets contact with other dogs is when I have friends or family over who have dogs, or the occasional trip to the dog park. In all those instances, the dog has been friendly. It does get REALLY excited upon first coming into contact with other dogs, and I presume that means my dog is just SUPER friendly, especially considering the excitement has always led to playfulness and companionship with the other dogs it's around.

The dog park is a great place to let your dog off-leash

I overlook the fact that when my family and friends bring their dogs over to my house, none of the dogs are on leashes, or if they are, it is only for a brief moment while walking in the door and the visiting dog is almost always unhooked or has its leash dropped upon entering my house.
I also overlook the fact that while visiting the dog park, my dog is not a leash and has all the space in the world to let off steam, determine when it wants to make physical contact with other dogs and has plenty of room to distance itself from dogs it does not want to be close to.
Now, here I am walking my dog at, let's say, a state recreation area. Rules dictate that all dogs be kept on a six-foot-leash at all times. In addition to this, my dog is not very good off-leash — he runs and runs and runs, doesn't always pay attention when I call him and sometimes, I've had to chase him through some pretty thick brush or swamp because he wouldn't come when I called him. I get nervous that he might run off and not come back, so I decide to keep him on leash.
All of the sudden, a dog who did get lucky enough to be let off leash by his owner comes tearing through the bushes at full speed — no doubt exactly what my dog wishes he was doing.
The dog slows down only a little bit, from full-tear to slow run, and comes right up to mine.
My dog is startled, first of all. We were upwind from this other dog, so my dog had no scent warning. Plus, the dog couldn't be seen until he emerged from the bushes, and by that point, he was right upon us. So, the only forewarning my dog had was some startling thrashing noise and then boom! Dog in his face.
The dogs go right into the butt-sniff mode and both their excitement levels are rapidly increasing. They are jumping around a bit and other than having to hold on for dear life to the leash, I think everything is going to be just fine.
But then, something happens that I didn't even see. All the sudden, the hair on my dog's back is up and he's lifted a lip; I can hear a low growl.
Before I even see this other dog's owner, the situation has turned sour and both dogs are now snarling and lunging at one another.
I pray the other dog owner arrives in time to help me pull the dogs apart before the injuries get so bad we both wind up at the emergency vet.

What happened?
A safe bet would be that the dogs didn't get enough time and space to know whether they really wanted to be that close to one another. They became too excited, too fast, and were too close together. My dog was on a leash and so, when he starting having doubts about wanting to be this close to the other dog, he had no option to physically remove himself. All he could do was warn the other dog — hackles raised, snarl — to get away. But the other dog didn't get away, and instead, responded to my dog's threats with his own threats. One decided not to back down and the other one couldn't run away, so they got into a fight instead.
This doesn't mean that either dog is bad or unfriendly.
The only thing that was bad was the situation the dogs were placed in while they met for the first time. Had the same two dogs been in a dog park, both off leash, this probably would've never happened. They may have even become pals and romped around a bit.
But the element of surprise combined with the extreme excitement combined with the leash-factor turned it all to shit.
And here's the thing — it doesn't take a bunch of things to go wrong for a bite or attack to happen. Just one thing. And sometimes, that one thing is just the leash factor.

Keep your dogs on leash
So here's the simple solution, folks — keep your dogs on leash in public places where it's required you keep them on leash.
Beside the mere fact that there are rules for a reason, there is also the fact that even if you want to risk breaking the rules, other people don't. Other people will have dogs on leashes, many of them are friendly dogs too, but as we've just found out, even friendly dogs are more apt to get into fights while on leash.
Adding to this whole idea of following the rules is the fact that there are people out there like me with dogs who are less than perfect. So long as we are responsible with our dogs — we keep them on a leash and we keep them away from you and your dogs — there is nothing wrong with us wanting to walk our less-than-perfect dogs. In fact, if our dogs are ever going to become better, we MUST get out there in the world to exercise our dogs and socialize them to different scenarios. Our less-than-perfect dogs will never become better dogs if we keep them locked up inside a house all day.
The ironic thing is, it is we-owners-of-less-than-perfect-dogs who are more likely to follow rules than those who own perfectly-friendly-happy-go-lucky dogs.
And one more thing — there are a whole lot of dog owners out there who own less-than-perfect dogs and don't realize it or even refuse to acknowledge it. Now that is worth being scared of, and it's certainly worth protecting your dog from.
Leashes — a cheap tool to prevent expensive emergency vet bills. Isn't that reason enough to use them?