The biggest mistake I see people make when greeting a dog is to rush in and touch the dog before the dog has a moment to figure out who they are or what their intentions are. This can be extremely dangerous when dealing with an unknown dog.
When approaching an unknown dog you should ALWAYS check with the person on the end of the leash. Make sure the dog is receptive, and DO NOT TAKE IT PERSONALLY if the handler tells you the dog is not. Just say thank you, and walk away. You never know what the dog’s history is. The dog may have been poorly socialized and fearful of people. The dog may have pain issues. The dog may have been rescued from a life of abuse. You just can’t know.
Not all wagging tails mean receptive dogs. Dogs wag for a multitude of reasons, not all of which are good. Do not take for granted that the dog is wagging because he is friendly.
Avoid staring at a dog that you do not know. This can be very threatening to a dog, and may make an otherwise receptive dog wary of you. If you want to be friendly, talk to the owner for a minute and give the dog a chance to become comfortable with your presence.
It’s never a bad idea to allow the dog to sniff you before you make any attempt to reach for him. This will help him feel secure that he has some information about the stranger who is about to touch him. Would you want a stranger to walk up to you and touch you without greeting you first? I wouldn’t. Sniffing is a greeting for dogs, so let him sniff you first.
Do not reach over a dog when you first meet it. Even if the owner gives their okay. The dog will appreciate your courtesy if you reach under his chin to scratch, and work up to the scratch on top of the head. Having anyone you don’t know who outweighs you tower over you can be intimidating. Be respectful.
I cannot stress enough how much scent means to dogs. As much as we rely on our eyes to maneuver and interpret the world in which we live, dogs use their noses to navigate their world. Imagine being blindfolded and asked to meet a room full of strangers. Don’t deprive the dog his most important sense. Let him sniff you!
Do not anthropomorphise the situation. The dog is a dog, and you are a human, and you have different cultural norms and ways of thinking and behaving. Contrary to popular thought, just because a dog isn’t comfortable with you does not make you a bad person. It also doesn’t make the dog a bad dog. There could be something about your body language, your scent, your haircut, hat, or what you are wearing that the dog doesn’t understand as friendly. It’s nothing personal, and you aren’t going to change it in a five minute interraction. Think of it this way: Human children have been known to be fearful of Santa, clowns, the Easter Bunny at the mall. It doesn’t mean that Santa isn’t a wonderful person. It just means that the child, using his strongest sense (sight), has found someone who looks very different then he is used to, and is scared of the unknown. And we don’t takefear personally, because it cannot be controlled by the fearful.