Caesar Milan just had a great post: 5 reasons why our dogs might bite us.
1) Maternal instincts. Always respect a mother dog with her young puppies.
2) Physical pain or discomfort. Dogs tend to be very stoic and adept at hiding their pain. If you inadvertently touch a sore spot, your pup may bite without warning. If you suspect this is the case, take your dog to the vet.
3) Fear. When your dog feels frightened or cornered, it may lash out. Frequently, this is a one-time event. But if there is a trigger (e.g., awaking your dog suddenly when it’s sleeping under the table), try to avoid that in the future.
4) Possessiveness. This occurs when we try to take a favorite toy — e.g., rawhide chewy — away from our dog. This may be characteristic of dominance, and if not corrected, turn into aggression.
5) Prey drive. Sometimes that tug-of-war game we love can lead to an unintended bite. In which case, it’s advisable to modify your play to something less aggressive, such as playing fetch with a tennis ball.
Since dogs manipulate their world primarily with their mouths, accidental bites can happen. That doesn’t mean that your dog is aggressive. Try to stay calm, and remember, always supervise canine play with young children.