Why are dogs distrustful of some strangers and not others?
Haven't you asked yourselves why dogs always chase postmen. What is it that postmen have that dogs dislike so much? Researchers say that this reaction has to do with previous negative experiences that come to the dogs mind. It could be the clothing, an intense smell, the way you look and the tone of voice with which you address the dog. Everything could have to do with the dog's reaction.
This type of selective memory could be very handy in the wild that is if you're willing to grow old. Dogs have been gifted with the ability to read evil minds and bad hearted people or animals but this doesn't mean they can't make mistakes, they also make mistakes.