Many of us are reserved when it comes to strangers, but we choose to trust some people we don’t know, because our intuition tells us we can trust them. Here’s why.
Studies have demonstrated that, although not trusting strangers is a vigilant action, we react differently to every person.

While we can’t trust some people, we choose to give a chance to others. It’s about the first impression they make or a kind of attraction, not matter of what nature.
And most of our reactions are usually based on personal experience. In our past, we had conflicts with certain people, they betrayed us.
We can associate them physically with people we have just met.
From our past experiences, we’ve formed an idea about how a mean teacher looks like, or we believe we can recognize someone who might cheat bases on their looks.
Our brain “recorded” the information and it keeps it in a form of a alarm signal.
Also, the information we get from a person (even if we don’t have a direct connection with him or her) becomes important to us if that person reminds us of someone we trusted.
We usually associate the received information with the features of the person sending it.
So, if we receive an inner alarm signal not to trust someone, the brain has a little stimulus-response map, and another person with similar feature will make us trust them or not, and we won’t know why.
Scientists have associated this form of brain programming with the response to a stimulus obtained by Pavlov in his experiments.
They say that if a dog was trained to salivate at the sound of a bell, knowing that he was going to receive food, the conditioning is kept at a lower level.
So, no matter what bell the dog will hear after some time, he will still salivate.