To ensure your dog's dental health, they require dental care on a regular basis. Otherwise they may develop problems.
Dog dental problems may also result from injury, foreign bodies such as porcupine quills or foxtail, malnutrition or systemic diseases which infect the mouth as well as other parts of the body.
Occasionally a puppy or kitten will retain some deciduous (baby) teeth after the permanent teeth have appeared. This may damage the soft tissues of the mouth and may even accelerate wear of permanent teeth. A veterinarian should be consulted to determine whether or not removal is necessary.
Pictured below is a 2yr old female chihuahua having her teeth cleaned and baby teeth pulled:

How to Care for your Dogs Teeth:
You should brush your dogs teeth once or twice a week. It is easier for your dog to be comfortable with teeth brushing, it is best to start a routine very young. You can use a small childs toothbrush or a brush from the vet, as well as an enzyme toothpaste (not human toothpaste). Next, scrub all surfaces of your dogs teeth, especially the back ones.
It is important for you to have your vet check your dogs teeth and gums, ideally during their yearly check up. They can check the health of the teeth,gums as well as for retained baby teeth or loose teeth. I recommend a yearly cleaning done by your vet to remove any plaque below the gum line.
Your dog relies on you for the health of their teeth. Please don't let them down, a healthy smile makes a happy dog for life.