A common misconception about dogs is they are naturally social animals without fail. It is true that dogs are born with social communication skills, but many dogs have to continue to practice these social skills by continuing to socialize with strange dogs in order for this “skill” to remain fresh. This is similar to learning a foreign language for humans, and if you stop practicing this language…..many loose the ability to communicate in that language. If a dog is acting aggressive toward a friendly dog, then this dog does not understand the other dog’s body language and friendly communication. A bouncy goofy dog approaching fast can seem like a major threat if they do not know how to recognize “bouncy, goofy, playful” signals and actions. The reason this same dog would play well with your mom’s dog is that “familiar” dogs no longer require social skills.
It seems logical that any dog should understand another dog (seeing that they are the same animal), but unfortunately that is not the case. If you think about it, what if another human came running, jumping and screaming at you in a foreign language you did not understand? Depending on your past experiences, you may be tempted to run or you may even punch them? You may perceive their actions as a potential threat, or you could recognize the joy in their face and learn that they just won the lottery. The point is, when we do not understand another’s intentions…we can perceive them wrong and often act inappropriately.