This article is from the Mastiffs Breed FAQ, by mmcbee@access.mountain.net (Mike McBee) with numerous contributions by others.
Mastiffs tend to react in predictable ways when faced with a
threatening person. If their owner is present and a tense
situation arises between the owner and a stranger, the dog
will usually get between the stranger and their owner, as a
sort of giant protective barrier that no sane mugger would
reach over. If the stranger does anything to escalate the
tension, the dog will probably growl or snarl at the person.
This may occur even within a family, if, for example, the
owners fight. This may upset the Mastiff greatly and inspire
him to protect the party who is on the receiving end of the
disagreement.
If a stranger breaks into a house where there is a Mastiff,
the Mastiff's tendency is to corner the person and not let
them get away, holding them until their owner gets home to
deal with the intruder. The dog may snarl or bark or even
snap at the intruder if he tries to get away, but usually
will not actually hurt him unless the intruder has tried to
hurt the dog or has succeeded in hurting him.
Dogs are creatures of habit, and it is this characteristic
that makes them good guards. If, while your Mastiff is a
puppy, you allow strangers like repairmen to come into your
house when you are away, the dog will see that as normal for
your household, and will not realize it is not "OK" for other
strange people to come in and do things.
Many Mastiffs, when mature, can recognize something about
people who have unpleasant motives, and are watchful or will
get in between you and that person. If your dog gets between
you and a stranger in a questionable situation, trust your
Mastiff! The dog may have sensed something you couldn't
recognize in that person.
Because of the intrinsic protective nature of the Mastiff,
training as an attack or guard dog is not necessary and to do
so may actually be detrimental to the temperament of the
Mastiff. Mastiffs are not suitable for attack training or dog
fighting and, if raised in kindness and socialized properly,
will be a strong, loving companion who will defend his home
and family if necessary.
 
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