Honestly, I think dogs, and cats both (well, most animals really) can sense fear, anger, you name it, in humans.
I groom dogs, and cats, for example. My husband, baths, on occasion for me. There are some dogs and cats, that will 'lash out irrationally' at him for no apparant reason, other than they seem to know he is afraid of them. When I go to pick them up, on the other hand, they show no anxiety, or threatening jestures. I think they just know who is afraid of them; I am not one of them!
I groomed a cat today, that one of the other groomers said is absolutely terrible; bites, claws...is just plain MEAN! Well...she hissed a few times, but other than that, I had NOT an ounce of trouble from that kitty...I figure it is mainly the demeanor difference between the other groomer and I; I am really calm, and will take my time and not subject the cats to extras (like the hi vo dryer), that tend to upset the cat further. I also don't raise my voice when they get 'loud' or 'viscous'...she does (saying "Stop that" or "Quit"), and I don't understand that...it's not like it accomplishes anything, other than further aggrivating an already finicky critter!Lol!