Bears, dogs, etc., are more sensitive to capsicum than humans. Thanks to this fact they can add more propellent and less active chemical to the can for back-country use. Also, wild animals are generally less determined to continue an assault due to any cognitive thought. Instinct/reflex might make a bear attack but once deterred does not have motive to continue. In other words, the attack was "nothing personal". In areas where rangers have sprayed problem bears a few times all a person has to do is to hold up a can of bear spray and let the bear SEE it to get the job done.
I have used pepper spray on dogs several times. Once I was fixing a flat and had a stupid and probably scared old lady let her dogs out to attack me. It has always worked and worked well. I am sure it has saved me from being bit. Once hit, most all dogs only need to see the can and the chase is over. I carry it all the time. I am too slow to outrun most dogs.