well i think they definately sense any fear or other emotion you may be feeling. and large powerful dogs seem to bring that out in people, especially if the dog considers himself the alpha & has never been made to see otherwise. thats why training & establishing your position as leader of the pack is so important early. i should also add that neutering goes a long way too, especially if you dont wait past about 6 months of age or so to do it. once they have the idea that theyre top dog, they can be a bit harder to deal with.
when i worked for the dog breeder we had a Malinois in for some show training. if you dont know, they are one of the belgian shepherd breeds becoming a little more popular in the US due to the increase in their use in protection work and as police dogs. theyre about the size of a german shepherd but he males tend to be a little more muscular (esp if not nuetered), almost but not quite like a pittbull. anyway, i had this dog out in the yard one day while i cleaned his run, then he refused to go back in when i was finished. well i did what i always did with the german shepherds when they wouldnt go where i wanted them to, got him by the scruff of the neck with both hands & started guiding him back to the run & he just kinda flipped out on me. my forearm looked kinda like hamburger when he was finished gnawing on it. they actually found a baby tooth in my arm when they cleaned it out at the hospital. he was only about a year old. BUT he had never been treated like anything but the alpha dog, and he sure didnt like it when i tried to assume that position. thats what his owner kept saying "but he's a dominant male"...plus he (owner) was kind of an ass about me even going to the hosp because he had to jump through hoops to keep the dog from going into quarantine at the pound....just goes to show ya how much of it really has to do with the owners & not so much the dog.