Accidents, disease and general wear can all make your dog lose teeth over time. If your dog has missing teeth, he can't eat just anything you give him -- you have to choose foods that are easy for him to break down in his mouth. Otherwise, eating can be uncomfortable and difficult.
Wet vs. Dry
Generally, there are two types of dog food: Dry and wet. Dry food like bagged kibble is crunchy, so it fosters dental health in a dog with a full set of teeth. If your dog is missing teeth, however, breaking down those crunchy kibbles can be difficult or even painful. A dog with missing teeth is better off eating wet food, which is soft and offers considerably less resistance. Consult with your veterinarian about the wet food that is healthiest for your dog, and about maintaining good dental health -- wet food can get stuck and linger in a dog's mouth, causing bacterial buildup and more dental problems. If your dog is already missing teeth and eating a wet food diet, you may need to adopt a teeth-cleaning regimen.