People are nice, why does my dog react like they all are a threat ?

Kindness is a moral perception by the human beings. It is subjective. Your dog perceives the world differently.

What he was meant for

Breeds were created and developed to meet human needs and desires. Some of them are close to humans, while others are more wary. Some will be vigilant towards strangers, while others will act as if everyone were their best friend.

Behaviors from discomfort to fear

To bear it out that your dog perceives all people as a threat, observing his/her body language, analyzing his/her motivations, and the reasons behind his/her behavior will enlighten you with certainty.

Some dogs will be tense outdoors and it can be difficult to identify the specific element – if there is one – that they are afraid of. Others will direct their discomfort towards certain individuals (men, women, children, screaming children, people wearing hoods, etc.) or towards individuals in a specific context (for example, people with canes in an elevator)..

Your dog may then adopt the following behaviors :

tail curled under his/her belly; looking with the whale eye; walking slowly; pretending to sniff or scratch or urinate; going around as much as possible, even walking on the road; barking loudly; growling; lunging; pulling in the opposite direction; hiding under furniture.

Generalizing

As far as their emotions are concerned, dogs can generalize: a chicken falling out of a baby stroller, all strollers are extra super awesome; a garbage can accidentally rolling over on his/her tail during collection, all garbage collectors are dangerous.

When I work to change the perception that some dogs have of teenagers, all I have to do is walk them (depending on the intensity of their discomfort) near where they are used to lunching and the dogs associate these young humans with interesting things.

If your dog belongs to a breed that is wary of strangers, if your dog has had aversive experiences, s/he automatically assesses humans s/he crosses or meets as potential threats, even before analyzing and understanding their intentions (or lack thereof) towards him/her.

If your dog deems every humans as a threat, her/his fear has become generalized. If your dog anticipates crossing or meeting humans and sees them as hazards for him/herself, s/he has become anxious.

Trying to reason with a fearful and/or anxious being, especially one who doesn't communicate with the same language as you, is deluding. Guiding them to change their perception of humans, modifying the outcoming behaviors, and supporting them when they can no longer tolerate their presence is what meets your dog's need for security.