Early socialization is important so the Mastiff knows who belongs in the house and who does not. For example, if a Mastiff puppy sees strangers coming and going all the time, they tend to see this as normal and might not recognize an intruder if they see one. However, a properly trained and socialized Mastiff will usually corner a burglar or intruder. Unless the intruder does something foolish, like try to hurt the dog, he or she will probably not be hurt.