I start brushing a dog's teeth while they are puppies to get them to accept the idea. At first you are just moving the finger-tip brush or toothbrush around their mouth uselessly, but gradually, the dogs accept actual brushing. Why bother? First, it helps to brush because the dog is used to you inspecting their mouth. You never know if your dog may have injured himself in his mouth if he wont let you look in there. Its also important to check your dog's gums and tongue if you suspect shock from an injury or bloat. If a dog isn't used to you inspecting his mouth, especially if he is in pain, he may react aggressively when you need to look in there urgently. Also, it does clean their teeth, even just superficially. To have a dog's teeth professionally cleaned means a lot of money especially due to anesthesia in larger breeds. Mine are 70 lbs and 135 lbs. I'd rather spend a few minutes cleaning their teeth a few times a week than spend thousands of dollars every few years having them put under anesthesia for dental treatment. I brush them every other day, gently, and alternate those days with days when they chew greenies or denta stix or similar. These guys also get buffalo bones, which are great chews that naturally clean the teeth and keep the gums in good health. And by the way, cats should have their teeth brushed too, for the same reasons. My cat was about 1 and a half when I got her and I SHOULD have started brushing her teeth, too. Her breath has always been fishy and now we suspect she has stomatitis. It's been treated, but it would have really helped if I had gotten her used to having me open her mouth to inspect it as now she must be sedated in order to check her for signs of infection.