The proper Mastiff should be a combination of grandeur and good nature, courage and docility. He’s always dignified, never shy or vicious.
A well-socialized Mastiff treats normal strangers with polite aloofness but will step between you and anyone or anything that seems threatening. Normally it’s not necessary, but if the threat continues, he’ll escalate his response as needed. Thieves who are foolish or unlucky enough to break into a home with a Mastiff will find themselves cornered until a family member arrives to call the police.
Mastiffs dislike conflict between family members as well and will step between arguing spouses or a parent punishing a child. While he might look tough on the outside, the Mastiff is a sensitive dog who can become shy, fearful, or aggressive if mishandled. Never treat your Mastiff roughly or allow anyone else, including children, to do so.
Temperament is affected by a number of factors, including heredity, training, and socialization. Puppies with nice temperaments are curious and playful, willing to approach people and be held by them. Choose the middle-of-the-road puppy, not the one who’s beating up his littermates or the one who’s hiding in the corner.
Always meet at least one of the parents — usually the mother is the one who’s available — to ensure that they have nice temperaments that you’re comfortable with. Meeting siblings or other relatives of the parents is also helpful for evaluating what a puppy will be like when he grows up.
Like every dog, Mastiffs need early socialization — exposure to many different people, sights, sounds, and experiences — when they’re young. Socialization helps ensure that your Mastiff puppy grows up to be a well-rounded dog.
Enrolling him in a puppy kindergarten class is a great start. Inviting visitors over regularly, and taking him to busy parks, stores that allow dogs, and on leisurely strolls to meet neighbors will also help him polish his social skills
Dog Time (2012) Mastiffs Retrieved from http://dogtime.com/dog-breeds/mastiff