This is PARTIALLY true but really its misleading a lot of people into adopting or purchasing certain dogs with the expectation that they can some how change this dogs base personality or temperament with certain treatments or training...
The conversation goes something like this;
Client: Hello, I just saved to Tibetan Mastiffs from being put down due to aggression and I need help with training.
Me: Oh wow, what types of problems are you having?
Client: Well I saved these dogs from their previous abusive situation and they are very aggressive, they are fine with me but they are very aggressive with any strangers on the property and I run a business from home and I need them to be friendly.
Me: Did you do any research on this breed before you adopted these dogs?
Client: I have had them now for two days and have done a lot of reading recently and finding that this can be a problem for this breed, but I really need it to stop, I would love to be able to take them to the dog park.
Me: This is a large guarding breed, they were not bred to be friendly with strangers and I fear that even with training they might always be an issue for you bringing strangers on the property... and I certainly would NOT have expectations of bringing them do a dog park... EVER!
Client: Well I believe its all in how you raise them
Me: We can maybe try some management techniques to try and minimize the danger to the people on your property... have you considered a muzzle?
Client: I tried that but I almost lost my arm.....
If you are not familiar with the Tibetan Mastiff please read about them here
All dogs have a genetic temperament, this is WHY we have different breeds that are bred to have a certain skill set. Take greyhounds for example, they are the fastest breed of dogs on earth, they got this way by selectively breeding the two fastest dogs for generations until they had a breed built for speed. The same goes for other types of temperament traits like guarding, personal protection, stamina, energy level.... really any set of skills or traits you can think of dogs and any other animal can be selectively bred for those traits so that each generation is better then the first. You can't make a tibetan mastiff friendly with every stranger just like you cant make a pug a guard dog... I mean you MIGHT be able to but the genetic odds are not in your favor here. Please do careful research on the type of breed you would like to own before jumping into a situation that might not be manageable for you. And please stop using the phrase "its all in how they are raised" because although this can make a difference depending on the individual dog its not the norm.
Just like we can breed Great Danes and Miniature poodles using selective breeding for this great size difference we can do the same thing with temperament traits, either magnifying them or minimizing them using selective breeding to achieve this. pay just as much (if not more) attention to the history and personality of a certain breed as you do for the physical characteristics like size, coat type, color.
If you need help selecting a breed or you have recently adopted a dog without a complete understanding of breed traits please contact me firstname.lastname@example.org