While any dog may have a severe reaction to strangers if he is not well-socialized, this is especially true of the Brussels Griffon. The Brussels Griffon has a naturally suspicious temperament, making him overly aware of strangers and dogs he doesn't know. When approached by a stranger or anyone not in his immediate family, the Brussels Griffon will likely react strongly; and if this behavior is not curbed, it could lead to more serious and detrimental behaviors.
The face of the Brussels Griffon is typically described as inquisitive, and that is exactly what he is, especially around people or animals he does not know. Unfortunately, unlike some kinds of dogs, the Brussels Griffon will not gently sniff a stranger or unknown dog. His reaction will be harsh barking and defensive behaviors. He is not a guard dog, nor should he be considered a good watch dog, but the Brussels Griffon will definitely let the family know when a stranger is approaching the house.
This barking can be a little irritating to a lot of owners, especially when the dog barks at people who are simply walking by the house, as the Brussels Griffon is likely to do. His suspicion may stem from his small size which makes him very vulnerable to people and animals who almost always out-weight and out-size him.
While the Brussels Griffon will recognize and love to spend time with those people he knows and loves, he will take his time getting used to strangers and other animals. With strange people, the Brussels Griffon will bark uncontrollably, especially if he has not been well-socialized as a puppy. With strange animals, like other dogs, the Brussels Griffon may become protective of his toys and food.
As with any dog, when the Brussels Griffon is allowed to develop and continue these suspicious behaviors they can become defensive and even aggressive behaviors. Aggressive behaviors like biting can develop, so be certain to curb your Brussels Griffon's aversion to strangers. He should not be allowed to bark at strangers in your home, and when he is a puppy, the best thing you can do is to allow him to be around other people and animals. This will help him to learn how to properly socialize and will get him used to strange people and animals.
The Brussels Griffon should never be allowed around small, strange children. Children naturally want to pet a puppy, and that should be fine with your supervision. But children are often loud and rambunctious. This kind of behavior can accidentally injure a small toy breed like the Brussels Griffon. Even if the dog is not injured badly by a child's behavior, he may become overly fearful of strangers in general and never be able to be properly socialized.