The answer to 'why does my dog....?' is usually very easily answered --- 'because it's a dog.' Much of what we regard as problem behaviour is quite often, as I explained in the Introduction, just normal dog behaviour exhibited in the wrong place and at the wrong time. My dog has just bitten somebody! Well, Dogs bite, don't they? My dog barks constantly! Well, dogs bark, don't they? My dog growls at me if I go near his food! Well, dogs growl, don't they? They also guard food.
All these behaviours, although totally unacceptable, are the thins dogs do. If you persist in being aggressive towards your do, the chances are that at some point it will become annoyed enough to bite you. What else can it do---send you a solicitor's letter?