Lots of people seem to have been trained to believe that any and every dog they encounter is JUST LIKE Ol' Shep. They do not consider that the strange dog could have been socialized in ways which mightn't comport with someone's naive ascriptions. Kids, especially if they have dogs at home, are prone to approach any dog as if it were "Fluffy" or "Muffy" or "Buffy." This is NOT a good idea.
This is a very useful video, and the accompanying text is straight-forward and easily understandable.