There is always a reason why a dog bites. It is a common manifestation of aggressive behavior. Puppies will bite if they have been poorly socialized or lack bite control skill. Some breeds of dogs have a greater potential for developing aggression than others - terriers, guard dogs, and hunting dogs.
Hyperactivity-Hypersensitivity Syndrome type aggression in young dogs and puppies is one of the most annoying behavioral problems since it commonly involves children. This problem develops when a puppy has not been disciplined enough by its mother or an adult dog in the pack and lacks the bite control. Usually these puppies or young dogs are not aggressive, but because they do not know when to stop and how to control their mouths, they can hurt a child or their owner. If a dog notices fear in humans, there is a real danger that these dogs may develop a dominance disorder which may be hard to correct.
You should teach your puppy what is called bite inhibition so that he learns not to bite. Every time the puppy touches you with its teeth, say "NO!" in a harsh tone of voice. This will probably not stop the puppy from mouthing, but over time should result in softer and gentler puppy biting.
Puppies who where physically abused during the first three months of their life, usually end up with behavioral problems as adults. One common result of abuse is fear aggression, in which a puppy or a dog will bite someone by whom he feels threatened. Fear aggression can also develop if a puppy or dog have been hurt: if a small child falls on your adult dog and sticks a finger in the dog's eye, you should not be surprised if the dog bites.
Fear aggression is relatively easy to correct. You should teach children in your family to engage in only positive activities with the dog, such as feeding, petting and walking and avoid rough play. Children should not give any corrections or negative feedback to dogs. Soon, the dog should feel confident that he can trust children and rely on them for many pleasant things.