The dog isn't the only one who pulls on the leash. People do it, too. We hold tension on the leash when passing by another dog or person because we don't want our dog to get too close. We pull our dogs by the leash to turn them or move them because it's faster than getting them to do it themselves. The leash becomes a crutch for leading our dog around because it's easy.
But, pulling our dog around by the leash causes problems.
Pulling on Leash
The natural reflex to a tight leash is to pull against it. Pulling on your dog's leash teaches them to pull in the opposite direction.
Reactivity and Aggression
Police Dog and Bite Sport Dog Trainers use tension on the collar to build drive in the dog to bite the decoy. Tension on the collar builds tension in the dog which can come out as barking, growling, and lunging at other dogs or people nearby.
If you do all the work for your dog by pushing, pulling, or dragging them by the leash, they have no reason to pay attention to you. They can tune you out and focus on all the interesting things around them instead. There is no thought or effort needed on the dog's part.
Dogs who are shy or anxious may shut down from fear. They may try to avoid you and be unable to follow cues because they are more sensitive to the tight leash than their more outgoing peers.
Shorten the leash. Don't tighten it.
Keep your leash arm relaxed.
Look for a smile in the leash.
Release tension quickly.