1. Every pull goes straight to his windpipe
When a dog pulls against a collar, all of that force lands on the front of the neck, right where his windpipe, the nerves and the blood vessels to his head run just under the skin.
Researchers who tested collars on a model of a dog's neck found that no collar kept pressure low enough to remove the risk of injury once the dog pulled. [1]
In another study, pressure inside dogs' eyes rose by about half when they pulled against a collar. On a harness, the rise was small and not significant. [2]
Now multiply that by every walk, every squirrel, every lunge, for years.