This article is from the Mastiffs Breed FAQ, by mmcbee@access.mountain.net (Mike McBee) with numerous contributions by others.
Mastiffs are excellent guard dogs. They go to the door and
bark, their hackles stand up, and they look formidable, but
Mastiffs, as a breed, are not trigger-happy. They have a
gentle, rather than an aggressive, nature.
Mastiffs need the company of their human family much more
than some other breeds of dogs do. A Mastiff left alone, tied
out, or kept in a fenced yard with too little human company,
will either pine away or develop destructive behaviors out of
loneliness and anxiety. Denied the needed time with its
family, a Mastiff may be much LESS protective because it
isn't sure it belongs to that family.
A normal, well adjusted Mastiff will protect it's family, but
only if the need arises. You don't want an aggressive Mastiff
that protects you from friends and family. The ideal
temperament is one where you never know that you are being
protected unless a true situation arises where a Mastiff's
services are needed.
 
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