I've never seen her do that before!
He's never bitten anyone before!
I don't know what made him do that!
He's usually not like that!
I can't believe she did that!
This is not like her!
He's always been so gentle!
Yes, sometimes dogs do behave unpredictably. Sometimes dog owners are clueless. Sometimes visitors deliberately provoke dogs--I've seen this happen, btw--and then cry fowl when the dog gives a very low warning growl.
I am a big believer in safety. This does not mean that the world must be made safe for me, but that I need to learn to behave safely, including around dogs.
There are dogs. You may never like them. You will probably always be very allergic to them and should avoid them. But the dogs do not know that you are allergic to them anymore than you know that your guests may be violently allergic to peanuts or tree nuts (and that almond furniture polish is so good!) or shellfish or that lovely tree that is in full bloom. Or to your cat.
There are bad dog owners. Most mean well but really don't consider the safety and well being of other people or their dogs when they fail to teach their dog basic manners (sit, wait to be greeted, no noses in crotches, etc.). Some of this is due to the same anthropomorphism that Tom Sawyer was engaging in (every time someone refers to themselves as their dog's 'mom' or 'dad' I just want to scream! Not that those people would teach their children to behave decently, either).
Most 'bad' dog owners are more along the lines of lazy or overly sentimental. Usually both.
But there are some bad dogs, dogs who are unstable and unsafe. There are dogs who are not placed in the correct environment: a dog who was bred for centuries to guard property is not an ideal family dog in a suburban setting, for example. A dog that likes to run and run and run is better served in a rural setting. A dog that likes to dig should not live with someone who wants a pristine golf course like yard. And so on. The last is usually only annoying to its owners while the first two can become menaces or are likely to be killed by a passing car.
There are some bad owners who intentionally keep dogs which are aggressive and are prone to attack. Usually trained to do this. These people should never be allowed to own dogs.
I'm not a perfect dog owner. My dog is noisier when people come to our house than I would like. She also gets enough mixed signals from the many, many people she encounters, some of whom gush and goo as soon as they see her and want to pet her (sometimes without asking if it is ok first) and some who will bolt when they see her at a distance (which isn't smart. Running triggers instinctual prey drive). She doesn't always know immediately who will behave which way with her and sometimes tries to be friendly (pushing her nose towards someone) when someone does not want to be near a dog. It's my fault if I allow her enough slack on the leash to permit this.