There is another myth that
really needs to die, unfortunately it isn't popular, dispelling this myth.
'There are no bad dogs/dogs are never born bad'.. etc.
Well.. yes, actually, many dogs are born 'bad', if by 'bad' we mean genetically predisposed to be fearful, to be anxious, to be more inclined to choose aggression when they feel threatened, to be more inclined to choose biting when they feel threatened...
There is a pervasive and horribly inaccurate idea that a puppy is a 'blank slate', and that being so, if you raise any puppy the right way, they will therefore turn out to be a wonderful, happy, well balanced dog.
That isn't true.
It cannot be true, because we now know that learned experience CAN be passed on genetically.
Fear (check out the mice and the cherry scent study, as its half 4 in the morning I can't be arsed to find it) learned by just ONE parent* can be passed to the offspring even when the offspring NEVER experiences the aversive stimuli.
So lets say Joe Toughs Pitbull learns to be terrified of the smell of baby powder - he's bred to Bob Arseholes pitbull who is genetically predisposed to want to use aggression first rather than last when scared.
The offspring all stand a chance of a/being scared of the smell of baby powder and b/being predisposed to use aggression first. But you wouldn't know which until much later on down the line.
No amount of training is going to alter that, you could raise that puppy with the absolute best of everything and still not ever guarantee that puppy won't react fearfully in that specific situation. If you're a sensible owner, the risks are minimal... but you could end up with a dog you have to manage super carefully it's entire life.
This is a wholly unpopular view with rescues of course because many of them raise puppies in less than ideal situations, from bitches and dogs who are unknown quantities and have (bitches at least) likely suffered some extreme stress during the pregnancy.
Its also typically a totally unknown thing as far as breeders of many dogs are concerned - neither old school showing breeders nor backyard arseholes tend to go to lectures on epigenetics.
So yes, unfortunately some dogs can even with the best intentions of breeder and buyer, and the best care and training - be born predisposed to be fearful.
If you combine that with poor training, use of aversives, bullshit like pack theory/rank reduction/dominance theory/poor housing, poor handling, lack of socialisation and enrichment... You easily end up with dogs who appear (the signs are generally there, but the people closest to the dog are neither looking for them nor likely to recognise them) to suddenly do something violent without warning.
We could seriously reduce the risks of this if no one bred from dogs with iffy temperaments, ensured bitches never experienced stress during pregnancy and raising a litter, ensured all dogs had the very best care... but I won't hold my breath for that!
(*study used males because theres a chance offspring can learn from the mother in the litter, but theres no reason to suggest they can't also get a fearful gene from the mother)