Does your dog behave differently when it is off leash then when it is on? Many dogs do. A lot of my clients have told me that their dogs are very playful and happy when they are off-leash but when they are leashed and other dogs are not, they get aggressive, defensive, and act differently.
That is why I get very concerned when I am walking a dog on a leash, ( which is the law), and a loose dog comes running out into our path. I can manage a dog on leash but I have no control over a loose dog that may intend to be playful when my dog is reacting like he/she wants blood. Off-leash parks and doggie beaches are wonderful for dogs to run loose. Enclosed back yards and fenced yards that are secure are wonderful for loose leash dogs. The city streets and apartment complexes open to other tenants, etc. are NOT the place for dogs to be loose. PLEASE, keep you dog on a leash. I don't care how well behaved and well-trained it is. It is a danger to other dogs and could possibly get bitten or worse.