Experts say that better dentistry can add years to the life of pets, and can spare them the pain that comes from poor dental hygiene. Some dogs undergo root canals, get crowns and even braces — and none of it is for cosmetic purposes. "Their teeth are their front hands," said Dr. Marty Becker, a veterinarian who is also co-author of Chicken Soup for the Horse Lover's Soul. "Try living without your front arms. And there's also the pain issue. A German shepherd puppy I knew was in so much pain, he had to sleep with a toy in his...