Dog Chemistry is an umbrella term for a dog's social skills, as it applies to both dog-to-dog and dog-to-human interactions. Chemistry is a very important factor in choosing a new dog. Since some dogs can be shy or aggressive toward people or other dogs, this can lead to bad encounters with strangers or unhappy feelings in the home.
A Shy Dog
Sometimes part of dog chemistry is being very afraid of a new person or other dogs. Dogs that often fear certain types of people (such as men, women, or children) may have suffered abuse by that type of person. Dogs that fear other types of dogs may have had similar trauma. The symptom of this fear is obvious: the dog will completely resist the pull on the leash and stay in one place. 
It is generally not recommended that the owner pull the leash harder in an effort to get the dog to "face the fear." After all, most humans don't get over their fears that way, so why should dogs? A dog needs to slowly transition out of the fear, and will slowly realize that he or she has no need to fear.
An Aggressive Dog
Of course, sometimes the opposite reaction can occur and the dog will run towards the other dog or person and go completely berserk. Even if they do not attack, they may bark, growl or raise their hackles. Generally, a dog is usually reacting out of fear more than anything else. In the dog world, the one who often looks the toughest is generally the alpha male. All the dog is trying to prove is that it can attack, not that it will. Even if the dog has never bit anyone, intervention is required by the owner in situations where the dog chemistry is aggressive.
Teaching Proper Social Skills
So how do you get a dog to interact with others successfully? It won't happen overnight and requires a slow transition process with planned social interactions. Most experts agree this works better on neutral territory, such as a quiet area of a park. For owners who struggle with dog-to-dog interactions, two dog owners can walk from opposite ends and pass within a distance from each other. As the dogs show less and less hostile signs, the gap between the owners can shrink, until the dogs can greet each other with a friendly sniff. This same therapy works for a dog who has adverse reactions to other people. Eventually the dog learns that there is nothing to fear and starts to be friendly.
