A three year old boy in California was recently attacked by a dog so severely that he required 50 stitches to his head and neck. What makes this particular incident different than many other attacks on children by dogs that we hear about is that the dog was on leash, with his owner, and the owner gave the child permission to approach and pet the dog.
Many times when people think of an "aggressive dog", they imagine a dog that behaves like Cujo; the dog snarls, growls, lunges and is ready to bite at any second. The actuality is that there really isn't any dog out there that behaves in this way continuously. It would be impossible for any animal to maintain that level of arousal and heightened adrenaline level every waking moment.
In real life there aren't aggressive dogs, but rather dogs that behave aggressively in certain situations. Most of the time "aggressive dogs" are well loved, well behaved family pets. They are affectionate with their owners, like to play and are trustworthy in most situations. Most dogs that behave aggressively do so in response to specific triggers (other dogs, strangers, when eating, etc.), and many times the aggressive reaction is due to the dog being afraid.
Reality makes it hard for strangers seeing these dogs only in situations that bring out the worst behavior to imagine that the dog can behave any other way. It's not uncommon for the owner of a reactive dog to have complete strangers make comments such as "Why would you want a dog like that?" or "Why don't you put an aggressive dog like that down?" It's hurtful to hear that a family member you cherish and love is considered to be so disposable by others.
And it also sometimes makes it hard for the families of these dogs to acknowledge that they have a dog who has a serious problem. When 95% of the time your dog behaves like the perfect canine member of the family, it's easy to be in denial about how serious his behavior is the other 5% of the time. And the stigma of having "an aggressive dog" makes it even harder to acknowledge the problem.
In the case I mentioned earlier, it later came out that this same dog had 2 previous, although less serious, incidents with other children about the same age as the three year old boy. I'm guessing that the owner was in denial as to his dog's issues with children, and unfortunately a little boy paid a huge price.
There are two morals to this story. The first is to not assume that when an owner gives permission to approach or pet his or her dog that it will be ok. The better option is to let the dog make the choice as to whether the interaction will happen. A dog who approaches is more likely (but not always) to want to engage with you, than a dog who doesn't move towards you or actively avoids you.
The second is that there is no shame in having a dog with problem behaviors. There is huge shame in not acknowledging the problems and doing what you can to keep everyone safe. Managing a dog to keep him out of situations that you know will make him uncomfortable is the first step. Finding a professional dog trainer or behaviorist to help you with these issues is the next.