The more time I spend working with people and their dogs the more apparent it becomes that sometimes common sense takes a back seat.
Here's an example: "My dog sometimes snaps at other dogs and seems uncomfortable". I advise the client to keep their dog away from other dogs while we work on the issues (in a controlled setting). This means not visiting off leash dog parks or allowing dogs to approach yours while on leash.
Within generally 1-7 days of the first meeting I get an email that goes something like this: "So we were at the dog park the other day and Rover bit another dog".
So why does this happen? I've come to a few conclusions here. I believe that dog owners inherently want their dog to be social and comfortable. They also believe that exposing their dog to other dogs will alleviate the anxiety and fear their dog feels.
Unfortunately that doesn't work very well when you aren't also working on counter conditioning to the fear and using dogs that are fairly neutral in their interactions with other dogs.
Dogs who have anxiety, aggression or even a high arousal around other dogs should not be meeting them in dog parks or on leash as this is likely to make the situation worse. Obviously keeping your dog completely separate isn't going to make things better either so here's what I suggest:
1. Contact a positive reinforcement trainer who has experience working with dog to dog aggression. The CPDT-KA trainers would be a great place to start but many cities have positive trainers who can help you out. Ask for references and watch them teach before you sign up.
2. Join a group class that keeps the dogs separate. Agility is NOT a good idea in this class because it increases arousal. I suggest an obedience class or rally obedience class.
3. Take your dog for walks where he/she can see other dogs at a distance and teach him/her to check back in with you rather than fixating on them.
4. If your dog is currently comfortable with a select dog (or dogs) then continue to let them interact in a fenced area where other stranger dogs can't join in. Do NOT introduce new dogs on your own but instead work closely with your trainer.
5. Pick up a book on dog behaviour. A few suggestions include "Help for your Fearful Dog" by Nicole Wilde or "Fight" by Jean Donaldson.
6. Do take your dog in for a vet check up especially if this issue is new. There are A LOT of medical reasons including pain, hypo-thyroidism and vaccine sensitivity that can create dog aggression. You'll often need to work with both a vet and a trainer to resolve it.
Please avoid taking your dog to areas where lots of dogs frequent because it can be overwhelming for your dog. This type of issue doesn't go away on it's own.