Dogs are man's best friend. Until they aren't.
Depending upon a dog's breed and how they're raised, they can be dangerous animals. There are an estimated 4.5-4.7 million dog bites on humans that occur every year in America. Of these, an estimated twenty or thirty result in death. The dogs on this list are considered to be some of the most potentially violent of all the breeds.
The Tibetan Mastiff is one of those breeds, and it's an old one. They were bred, like all shepherds, to guard livestock. The tribal people of Himachai Pradesh also used them as guard dogs, to ward off leopards and wolves. As you could imagine, they have the potential to be fierce animals.
Tibetan Mastiffs benefit from clear, well-enforced boundaries. It's also good to socialize them with other dogs when they're puppies and to keep them well-exercised.