If nobody ever puts a collar and leash on a dog, there's no way it can learn to pull on the leash. So, if a dog pulls on the leash, the owner must have taught it to do so. Keep in mind that what a teacher does is makes it easy to learn. Now, pulling on the leash was most likely not what the owner intended to teach, but, inadvertently, that's what was taught.
Typically, this is how it happens. A leash and collar (likely a flat buckle type) are put on the puppy for the first time. Unaccustomed to the restraint, the puppy pulls on the collar, sometimes almost choking himself. The owner, fearful that the puppy will actually harm himself, lets some slack into the leash. What he intends to do is relieve the stress on the puppy, but what he actually does is teach him that, when he pulls, the pressure is removed from the collar.
Within a few seconds the puppy is back at the end of the leash, coughing and sputtering worse than before. If the pressure is not immediately removed from the collar as he expected, he pulls harder until the owner finally relieves the pressure again. Now he's more convinced than before that pulling on the leash is the way to escape the pressure. These dogs usually wind up, after a while, towing their owners all over the place.
Far better to stand still until the dog puts slack in the leash himself and learns not to pull. If you're afraid he won't figure out how to escape the pressure himself and may actually hurt himself, think how many dogs have been tied to a dog house or a tree. I've never known one that didn't learn where the end of the line was. I'm against tying dogs to trees except in an emergency, but most dogs are not stupid or self-destructive, and the good thing about trees is, they don't make the kind of mistakes we humans do.
Good luck with your training!
Neil