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The Root of the Behavior
This all being said, you probably shouldn't touch it. Your hands carry a lot of bacteria, oils, and dirt. Best case, you cloud their sense of smell with the oils from your hands. Worst case, those sensitive mucus glands absorb some harmful bacteria and you cause an infection. These kinds of things can be easily avoided if you simply don't touch their nose. A canine who doesn't want to be touched, but is anyway, could react aggressively. This, again, is a built in defense mechanism. With the help of some advanced training, some dogs can be taught to allow their noses to be touched. This kind of training is most commonly seen in support dogs or therapy dogs, as the owner may not have the mental capacity to know better. This type of anti-aggression training can be quite complicated and require a professional trainer and hours of work. Again, simply not touching your dogs nose is going to be your best plan of action.
Encouraging the Behavior
Your dog will express any discomfort they may have to you through their body language. If you want to know how your dog feels about any given situation, take note of this body language and they will make it apparent to you. Commonly dogs will stiffen, whimper, and pull back from situations that put them in discomfort. These are natural responses to this situation and do not indicate any form of aggression. You should start to worry when they bare their teeth or growl at a low pitch. These are the most common indicators that your canine is about to attack or exhibit some form of aggressive behavior. If they do this, immediately cease your action.