You have a dog who is friendly all of the time.  You clip his dog leash on and he suddenly becomes a monster!  There are lots of explanations for this kind of behavior but it could be a sign of frustration. Your dog is used to expressing himself by the freedom that he is given. When on a leash, his movement is limited and he no longer has the freedom that he enjoys so he gets frustrated.  Proper socialization as a puppy is a key to preventing leash aggression.  Don’t worry, even learned behavior can be changed.  Pulling on the leash and speaking to your dog will only re-enforce the unwanted behavior. You have to teach alternative behavior and utilize the help of a professional. A professional will give you the proper training tips on how to improve his behavior.   If the pulling is really bad, you should consider a  harness or a halter. This is much better for you and your dog. Pulling on a leash hurts your dogs neck and can cause serious problems down the road. A halter will give you more control and your dog will not pull as much. A collar chokes the dog and causes him to pull more. A harness will make things much easier for you.   Good luck, this could be a tough one if your dog wasn’t trained as a puppy on how to behave when his leash is on.