Ever felt that your dog was walking you, instead of you walking your dog?
Nobody likes to be dragged along by an unruly pooch on what is supposed to be a pleasurable walk, and it is dangerous too. A dog that constantly pulls on the lead can cause damage to their neck and windpipe from the constant pressure, or jerking movements, or can cause their owner to accidentally let go of the lead, freeing them to run, possibly into traffic or another hazard. Also, a dog that constantly pulls on the lead, especially if it is a large dog, can overpower and pull their owner over, or into a hazard. Training your dog not to pull on a leash is important for their safety and yours, not to mention making your walks together far more pleasant!
Why do dogs pull? Dogs are naturally inclined to investigate and explore with their eyes, ears, and most of all, their nose. Following scents, like other dogs, prey, or people can cause your dog to constantly be pulling on the lead, as he seeks more freedom to check out what he smells.
Training your dog not to pull on the leash can be difficult because his natural reaction to restraint is to pull. This is an instinctual survival method--if your dog is caught on something in the wild they tend to fight it to get away. Teaching your dog not to pull on his lead will mean providing an alternate outlet for his natural tendencies, and an alternate reward for his non-pulling behaviors. Dogs also like to trot, which is a more natural gait for them, so giving your dog some nose time and trotting time can be a reward for him while you are training him not to pull on the lead.
Teaching your dog not to pull on their lead can include teaching the 'heel' command; the dog walks quietly on the owner's left side, even with the owner's leg, or may just involve your dog learning to walk with slack on the lead, stopping when you do. Either behavior will prevent your dog from pulling the lead and hurting himself... or you.