Think about a recent situation which involved you seeing another owner walking towards you and your dog. What was your reaction? Did you hold your breath for a moment? Maybe you tightened your grip on the lead? Probably, you even pulled the lead a bit or started talking to your dog. Chances are, you did all three and something else, if you were walking a dog that is reactive on lead. The behaviours are not themselves a problem. The issues arise, when your dog learns to associate them with other dogs approaching him. After that you are trapped in a vicious cycle:
In addition to your behaviour and body language, there are other possible triggers for your dog. A common trigger is a certain type of dogs. For example, your dog has been attacked by a large white dog and will display aggression towards similar looking dogs. Another possible trigger is a certain place. For example, your dog may only react to other dogs on a narrow pathway or on a certain street corner.