Mostly, it's because they are strange. They are warning you the some one is coming into their territory, might even be saying "Hi, come over and pet me". I get he same walking down the street, in jeans and a t-shirt. As most of the dogs are fenced in, as I approach, I can tell if the dog just wants attention, or will rip my arm off if I try to pet it. Mail Carriers are trained to treat every dog as a possible threat, and don't try to get friendly.