6 month old cocker, barking and possible aggression

Question
I have a 6 month old male Cocker Spaniel. The first few months  he was very friendly to strangers and would let anyone pet him.  But recently I have noticed a change in his behavior.  When I walk him or take him to a public place where there are lots of people around he barks at everyone and everything.  When someone tries to pet him he jumps back and starts barking at them.  I can always tell when he is getting ready to bark because his tail stiffens and points straight, he takes a dominate stance and starts barking without approaching who he is barking at. He doesn't seem like he wants to bite them but almost seems as if he is afraid of them.  I've took him to the dog park several times.  He almost always seems to get in a fight with another dog.  Sometimes he starts chasing other dogs barking at them only to have them attack him.  But once he is attacked he tries to retreat.  After being at the park for a while he tends to calm down and becomes less aggressive. However, he loves certain people and certain dogs and acts completely different around them. He is very smart and doesn't seem like he would purposely cause an altercation with anyone. Could this just be a phase he is going through? Or should I be concerned?

Answer
Learned dog behaviorists tend to use the term fear stage, not phase.  Dogs do pass through periods where they show abnormal amounts of fear.  I would try to reduce his contact with strangers for the next month.  Concentrate on obedience and leadership which could also be part of the problem.  It could be he now thinks he is a big boy and it is his job to protect you.   The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm