Dominance issues.
A dominant dog thinks that he--not his person--is in charge, and is leader of his family pack. These are often the dogs who "unexpectedly" bite people--usually their own family, usually children, and quite often in the face--the dogs who "were always friendly" until the day the human did something the dog disapproved of. The 'offence' varies, but most often the person corrected the dog, grabbed him by the collar, removed him from the bed or couch, or (in the case of a child) threw their arms around his neck in a hug or draped themselves over his back...both of which are dominant behaviours, that a dominance-confused dog who thinks HE is in charge may not accept.
Dominant dogs are quite easy to spot by people well versed in canine body language and behaviour. However, casual owners may not notice this
"disaster in the making"--these dogs are generally confident, cheerful, fun dogs until their "subordinate humans" break one of the rules of canine etiquette...then it is the dog's job to correct his subordinates in the way dogs do: with teeth. When corrected, a puppy or subordinate dog will roll over, stop moving, avert his eyes, and/or urinate on himself. Conversely, a human child will likely fight back--thrash, scream, and otherwise do things that convinces the dog that it *still* is not showing proper submission, and more force is required! Dogs who have developed a dominance problem are a great risk, especially to children. Note that they have DEVELOPED a dominance problem. This is a direct result of how the animal was raised, and like any other behavioural problem, can almost always be repaired with proper re-training. However, if the owner is unable or unwilling to rehabilitate the dog (and prevent him from biting anyone in the meantime!), the animal is best rehomed with a professional, or put to sleep.

When their option to avoid a threat is taken away, even the meekest dogs are likely to take the only remaining alternative: a bite.
-Never corner a dog, nor reach into an enclosed place to grab him (especially in anger), or grab him roughly by the collar (or any body parts) when he is showing signs of fear or distress.
-Don't lean or hover over a frightened dog--instead, use non-threatening body language and avoid staring at him. If you scare him badly enough, he
will bite--it's the only choice you have left for him.
-Children can be very intimidating to shy or fearful dogs, and are liable to be bitten if allowed to climb on or otherwise harass the dog.
-Never leave a dog unattended on a chain, where he can be threatened and unable to escape. (Ideally, dogs would not be chained out at all.)
The warning signs of fear are obvious to anyone who cares enough to
look at a few diagrams or photos, and fear bites are easily avoided by not "pushing" the dog past its breaking point.
Caution: do not turn your back on a fearful dog. A rare few have learned that a sneaky bite to the hamstrings will drive the scary person away--but they don't have the confidence for a full frontal challenge.

The vast majority of dog attacks are committed by intact males. Neutering your dog will reduce his hormone level, resulting in less desire to defend his territory, attack other males, and be protective of females in heat. Spaying your female dog eliminates a major catalyst for male aggression (guarding a female in season). Females can also be fiercely protective of pups, and may attack any perceived threat to her babies.

Resource Guarding.
Many dogs who were not properly desensitized to handling of food and high value objects as pups (or who suffered from hunger or neglect) will defend these things from humans.  Dominant dogs may also believe they have the "right" to guard their food or other objects. Disturbing such a dog when he is in possession of food, treats, toys, or even his water bowl or a favorite spot on the couch, can result in a bite. This dog needs to be retrained and/or desensitized, and probably put on a rank reduction/"No Free Lunch" program immediately...but in the meantime, touching "his" things is a bite risk.

Some dogs feel the need to protect their owners from any (real or perceived) threat. If your dog is of this mindset, extreme caution is a must when there is any interaction with the public. These dogs need to be securely fenced at all times, or attached by leash to a person strong enough to control the dog (and who is aware of its potential).
Many dogs will also "protect" their territory (yard, car, or house) and caution should always be used when entering a property with a dog.

Some undesirables actually encourage, praise, and reward random aggression towards strangers, and
train this behaviour into their dogs. Not much can be said here, except that is is not the DOG who is the primary menace to society. :-(

Continue on to More reasons for bites, and More on Bite Prevention.
Why Dogs Bite.
(YES, there is always a reason! Yes, an experienced person can "see it coming". Yes, YOU can prevent Dog Bites!  Read on, to find the most common reasons for bites.)