It has EVERYTHING to do with breeding. And training, of course.
Staffys, Pitbulls, those types... you name it, they were bred for a reason - as aggressive dogs whether for fighting, protections, etc.
As breeders, ppl tend to try and weed out the more aggressive traits a breed may have, but you will always have throwbacks, and furthermore, you will always have irresponsible owners who do not try to curb a dogs temperament.
Dogs SHOULD come with a handbook - like a car. But we'll never be so lucky unfortunately. Ppl will buy a certain breed - lets say a 'pretty, good looknig husky' or malamute or eskimo dog, whatever... simply because they like the look of the dog, without knowing theyre all capable of bulk pack behaviour and can be snappy aggressive bastards at times.
Im well aware that as an owner of a 'herding' working type dog (bluey) that he has it in him to deliver a pretty nasty bite or nip even though he is a very friendly, happy dog.
So bascially yeah, breeding, instinct and all that has plenty to do with it but also the lack of training/handling and knowledge on owner's parts too, all impact on the type/temperament a dog ends up like when it is grown...
*lecture off* heheheh