Dogs sometimes pull on the leash because the dog walks at a faster pace than the handler. Sometimes dogs have been inadvertently rewarded for pulling (like when they see someone they like and pull toward them to greet the person). Not only is this often uncomfortable for the handler and presumably the dog, but collar pressure on the neck has been linked to several health issues in dogs.
To teach a dog to walk with the leash loose, pick an area where there are few distractions and have the dog on a flat buckle collar and 6-foot leash. Holding the leash with the right hand (so the left hand is free), and with the dog walking on the left of the handler, begin to walk.
If the dog starts to move ahead of the handler, the handler can change direction gently, letting the dog know they are changing direction by making a clapping sound with their hand clapping against the handler’s leg to get the dog’s attention or they can gently change direction. When the dog is walking on a loose leash, praise the dog or provide a food reward from the hand closest to the dog, while walking.
If there isn’t room to change direction and the dog is pulling, simply stop and wait for the dog to return to the handler and then proceed. With consistency, dogs will start to adjust their pace as soon as they feel pressure on the neck. As the dog understands the exercise, distractions can gradually be introduced and the dog rewarded for walking on a loose leash despite the distractions. There are also several humane harnesses on the market that help slow a dog down. They are available at most local pet stores.