You've probably noticed a common pattern: dogs try to instantly establish friendly relations with some people, while barking at others for no apparent reason.
However, dogs have reasons to form their opinions about humans.
The dog instantly "reads" gestures and facial expressions, so it will figure out the person it doesn't like. The animal won't feel sympathy for him either.
With fear it is a little more complicated: if the dog is prone to dominance, then it will try to scare the person even more.
Sometimes we don't even realize how strong the smells we are used to using can be. For example, animals may be negatively disposed towards household chemicals sellers because their clothes and hair have a specific aroma.
For the same reason, a dog may not like mailmen: dogs do not like the smell of printing ink that they come into contact with.
For dogs, raising their voice is a sign of aggression and dominance. If someone starts talking loudly, laughing, or shouting in your presence, the dog may "express displeasure."