Dogs who are provided a stable environment and good socialization training learn from puppies where they fit in the society of other dogs and among the humans in their life. Like humans they are more comfortable and behave better when they enjoy healthy levels of confidence and understanding of what is expected of them.
Also like us they come to regard certain areas and objects as their own and can become overly possessive of these things. This should always be discouraged as its one of the quickest ways to develop problems with aggressive behaviors. There should be no doubt as to who is the pack leader.
Another trait we share with our dogs is the tendency to become less patient or tolerant as we grow older. It's not uncommon for an older dog to become snappy or even seriously aggressive with puppies or children even if they were extremely tolerant in earlier times.
Loss of acuity in hearing and eyesight adds to their confusion and makes them still less likely to tolerate confusing or unpredictable situations. As dogs age they may become less mobile and find it hard to keep up or assert their claim to their place in the hierarchy. This can be stressful and cause a normally friendly dog to behave more aggressively than he had before.
Many serious aggression issues arise from situations of neglect and abuse. Remember that your dog is dependent on you for virtually everything. Take the time and trouble to see that his needs are met and he can count on you for support and reassurance in times of stress. He is a part of the family.
where they fit in the society of other dogs and among the humans in their life. Like humans they are more comfortable and behave better when they enjoy healthy levels of confidence and understanding of what is expected of them. 
