That is mostly true. But some breeds or types are more reactive by nature or have inherited higher prey drives. While some gentle giants would be very difficult to make aggressive even if that was your goal.

For instance, many terriers have been known to be more reactive with dog aggression just under the surface. The American Pit Bull Terrier (APBT) is actually less so than many terriers, yet its size and strength make it far more dangerous IF it is aggressive than the Cairn Terrier even if the latter were far more aggressive. One breed I have known of no individuals in which were fully trustworthy is the Cane Corso (pronounced CA nay COR so) And they are far larger and more powerful than APBTs. But if breeders select for less aggression, over time they may become one of the gentle giants.

I have had three different Cane Corsos lunge at dogs I am training as guide dogs over the past few years. And at a UKC dog show there was a Cane Corso which I was told was gentle and totally trustworthy. It seemed to be the case as the day progressed
until while being walked by its owner it suddenly pulled away from her and attacked a Standard Poodle who was being primped on a g***ming table. The Cane Corso shook it like a rag doll. At another show a group of 5 APBTs busted out of their RV and attacked me and two of my German Shepherd pups. (I was the only one hurt as I tried to protect my pups while several people charged in to help) But since I have worked with many APBTs who were friendly, calm, and gentle; I don't blame the breed. But it isn't just how they are raised. Some bloodlines are more prone to aggression than others, as are some breeds.

I have been a guide dog trainer since 1968. And have trained many service dogs since 1980. And pet dogs as well as offering free dog classes to 4Hers.

My daughter has one that's about 8 months old now. She also has an American Bulldog that is an absolute truck!
She had the ears clipped on the Cane Corso, and I don't like that, but it's not my business. He is a sweet, huge, goofy bundle of slobber.
He is very well disciplined, and behaved for a pup his age.
I just don't like his ears.
Ive always thought that you can kind of read a dog by their ears. Like with a horse.
Having them cropped leaves nothing there.
Just my opinion.

My daughter has one that's about 8 months old now. She also has an American Bulldog that is an absolute truck!
She had the ears clipped on the Cane Corso, and I don't like that, but it's not my business. He is a sweet, huge, goofy bundle of slobber.
He is very well disciplined, and behaved for a pup his age.
I just don't like his ears.
Ive always thought that you can kind of read a dog by their ears. Like with a horse.
Having them cropped leaves nothing there.
Just my opinion.