Why the neck takes the brunt of the pressure
Every time a dog lunges forward against a collar, the force lands directly on the trachea and the surrounding soft tissue of the neck. For most dogs an occasional tug isn't a big deal, but dogs who pull hard and often, especially small breeds, flat-faced breeds, and dogs with any history of coughing or a collapsing trachea, are the ones where a collar-only setup becomes a real concern. A harness moves that same force onto the chest and shoulders, which is simply better padded and better able to absorb it.
Front-clip vs back-clip harnesses
Not all harnesses do the same job. A back-clip harness (the leash attaches between the shoulder blades) is comfortable but does very little to discourage pulling, in fact it can make a strong dog feel like a sled dog, since pulling into the harness is a natural, efficient posture. A front-clip harness (the leash attaches at the chest) gently steers the dog sideways when they surge forward, which interrupts the pulling motion without any pain or force. Many trainers recommend a dual-clip harness so you can use the front ring for walks in distracting places and the back ring for calmer, well-mannered strolls.
When a collar is still the right call
A flat collar is still useful and appropriate for a dog that already walks politely, for holding ID tags, and for very short, controlled moments like getting in and out of the car. The issue isn't collars as a category, it's using a collar as the main point of leash control on a dog who pulls hard, day after day. If your dog is a light, occasional puller, a well-fitted collar plus consistent training may be all you need.
Equipment is a management tool, not a fix
Switching to a harness will usually make walks more comfortable immediately, but it won't teach your dog not to pull. Pulling is a learned habit, dogs pull because moving forward, sniffing that lamp post, or reaching another dog is rewarding, and the pulling behaviour gets reinforced every time it works. Loose-leash training (rewarding your dog for walking near you, stopping when the leash goes tight, and practising in low-distraction environments first) is what actually changes the behaviour long-term. Think of the harness as removing the immediate physical risk while you work on the training underneath it.
Getting the fit right matters as much as the style
Whatever you choose, a poorly fitted harness or collar can undo the benefits. A harness that's too loose lets a determined dog back out of it or twist to the side; one that's too tight can rub and chafe at the armpits. A collar should sit snug enough that it can't slip over the head but loose enough to fit two fingers underneath comfortably. If you're unsure which setup suits your dog's size, strength, and any existing neck or back issues, a vet or a certified trainer can look at your dog in person and recommend the safest option.
