A lot of owners humanise their dogs. They forget that they are pack animals and their needs to be a hierarchy.
This is why small dogs are nasty little gits...
Oh he's so cute I cant tell him off for pinching food off your plate.
Hes not pinching food hes claiming it as his own because the owners have let him become the pack leader.
I'm pretty sure the "pack leader" theory has long been debunked, but that's not to say people shouldn't demonstrate to their dogs that they are in charge in the right way.
I think in general, a lot of people are pretty clueless about dogs and dog ownership. I include dog owners and non-dog owners in that. Foremost, people have to remember that any animal can act unpredictably, regardless of how domesticated they are, and treat them with due respect. There are also a number of irresponsible dog owners who do not work with their dogs to improve their behavioural issues, and also don't mitigate the risk they face to the public/other animals. Worse still, there are owners who knowingly breed and nurture dangerous, highly reactive dogs. I assume it is a status thing for some people.
In our case, we have a nervous rescue greyhound called Barley. He came to us after being beaten and starved because he wouldn't race. He's the gentlest, sweetest dog 99% of the time but the fear that has been ingrained in him by his previous mistreatment will never go away and we have to work with him on that. His behaviour is actually remarkably predictable. He has very specific triggers (people looming over him when he's lying down, and people bothering him in his bed) and he will sometimes react by biting or snapping. I've been caught out by this a few times but in every situation it's been my fault for overstepping boundaries and being complacent.
For our and other's safety, we muzzle him in public and keep him on a lead around other dogs (even though he's not dog/animal reactive). We get funny looks for muzzling him, and the overriding presumption is a muzzle means the dog is dangerous. However low the risks are, we do it for the safety of the idiots who think they can just waltz up uninvited and crowd him, or worse let their kids do the same even when we tell them not to. There seems to be a presumption that "awwwww the cute doggy" even when the person that knows the dog best is telling you what to do.
I know there will be people in this thread that are of the opinion that any dog that shows signs of aggression is not safe, but Barley was born into a life he clearly wasn't cut out for and was treated horrendously by the people who were supposed to care for him. He deserves the chance of a happy, fulfilling life. Unfortunately, some dogs are let down by their owners, deliberately or otherwise. It's so difficult to work out how you legislate for this without impacting the majority of responsible owners, particularly given the sorts of people who will breed/own dangerous dogs are unlikely to be put off by licensing or other requirements.