There are no bad dogs, only bad pack leaders

Your dog picks up on your energy. High pitched, squeaky voices, angry yelling or soft, weak directions are all varying vocal tones a dog can become conditioned to react to. That sounds a little confusing so I'll give you an example.
The first time your dog heard angry yelling he came out to see what was happening and in the heat of the moment you or another person a) chased him away aggressively b) yelled at him or c) everyone stormed away and your dog was left alone.

In your dogs head now: angry yelling = negative outcome.

Here's another example: you come home from work and open the door. Happy to see your dog you speak in a high-pitched, excited voice and he jumps up on you to say hello also visibly excited.

In dog language: high-pitched voice = jump up on person.

Now of course when someone comes to your house that is scared of dogs they may walk in, see your dog and speak in a high-pitched, squeaky voice to say "please take your dog away from me!" and what will your dog do?? He'll jump up on the person because that's what he thinks he's supposed to do. Then you react aggresively to move your dog off the guest, most probably put on a stern voice to scold him for jumping up, so he slinks away as he's conditioned to do.

Can you see the confusion for your dog? Dogs don't think so much as they are conditiond by events, action = response, throughout their puppyhood. There can only be one outcome for one action and this needs to be consistently reinforced through continual training and practice.