Don't touch my dog unless you've asked first
It's a common sight in the U.S. — people walking down the street, in the park, heck, even in some pet-friendly stores with their dogs. But all too often people assume it's OK to come up and just pet my dog. And while I know you mean well, I'm really not OK with that.
You'd never approach a dog alone on the street, but why is it acceptable behavior to just touch a dog because it's with an owner? You really need to be following "strange dog" rules with every dog you meet — because any dog can bite, no matter how sweet it seems.
It isn't just bad dogs that bite
Dogs are individuals, just like we are. Even if you've had 20 dogs in your lifetime, you can't possibly know all (or any) of my dog's peccadillos. You don't know his history, his fears, his anxieties. You never know how a dog is going to react to being approached, or what it is about you or how you walk up that might set the dog off.
My dog was abused. We were told that when we got him, and I'll admit, Mosby was a fear biter when we first brought him home. I'm sure now that a lot of you must be thinking, "Well, then don't bring him out." Fair point but exactly where is he supposed to pee?
Professional dog trainer Susan Carol notes, "Dogs can't ask us to please leave them alone. They can't tell us that something's bothering them or that they need some space." Fear biters don't do it because they're aggressive, they do it because they feel it's the last way they have to communicate that they're panicked and uncomfortable.
Dogs have anxiety triggers, too
Some dogs, especially small dogs, are put off by kids. But for some dogs, there's an inherent anxiety because they know what happens when kids approach them.
Mosby used to have a thing about work boots. Seriously. Even people he knew and had been friendly with were suddenly evil when wearing them. We assume it was because the person who abused him wore them, but any dog could have a trigger like that (just like any human could).
Owners may know things about their dogs that you may not. But if you ask first, they can warn you.
Stay out of my personal space
You wouldn't walk up to me and start rubbing my neck, would you? That's a little creepy, and dogs may feel the same way, and their rules about "OK touching" may not be what you think they are. Dogs don't like being pet on the head or face. Even your own dog probably just tolerates it because you're the alpha (or you get bitten, which is another issue altogether). Dogs don't like hugs (which is what your kids may well do if they see you model bad behavior by approaching a strange dog without asking).
You wouldn't react well to a stranger doing either of those to you. As you might imagine, any animal might be a little weird about another animal going for its eyes, ears or nose, which it may need for survival. Some dogs will grin and bear it, so you (probably) won't get bit, but you could be inadvertently conditioning a dog someone is training to fear strangers or even just startle a normally docile animal.
The risk you're taking isn't yours
In the U.S., dogs are considered property. You wouldn't walk into a house and look around just because you thought it was cute, right? The difference between a house and a dog is that dogs can bite back. And if my dog does bite you, I can be on the hook for your medical bills (and possibly more depending on the circumstances). And the law may not take into account whether you asked my permission to touch my property or not. In fact, they probably won't. If you walk onto my property without permission and get hurt, you still may have a case against me.
But more important, the reality is, if a dog bites you, you could be playing with a life. Depending on what state you live in, dog bite laws generally limit the number of "bites" a dog can have before the dog is put to death. If a dog is classified as dangerous (i.e., a pit bull), that number sometimes goes down.
Most dogs don't come anywhere near that number in their lifetimes, but I'd hope if nothing else here means anything, you're at least not willing to potentially risk a life just to scratch a dog's ears.