Aggresive behaviour

Question
My 2 year old,neutered, male labradoodle has started showing aggressive behaviour towards people and dogs he is meeting for the first time. He's healthy and gentle with those he knows but sometimes acts hostile towards unknown dogs and people.  What is the best way to reprimand this
action and change this behaviour.  Note that he has always been well socialized and plays well with most dogs & people.

Answer
Hi Todd,
First, when dealing with "aggression", never punish the dog, as you could make things worse.  You don't want him to associate people and dogs with negatives happening to him.  In fact, if you are using any pain-inducing collar to walk him - I know how some large dogs can pull - please give that up, and use an Easy Walk or Sense-ation harness instead (or a Gentle Leader type head collar, if you prefer).  At age two, he has reached the age of social maturity, and may be trying to sort out where he fits in his own social group.  You need to be the benevolent leader of that group, so that he has confidence in you, and that you will protect him and so that he looks to you for guidance.  One thing that I would want to know is whether this so called aggression is happening on leash or off, or in what context.  Some dogs are simply "leash reactive" and others are truly guardy or aggressive - if your dog is socially appropriate off leash with unfamiliar people and dogs, but is starting to become barky and lunging on leash, reactivity is the more common diagnosis.  
Regardless, the object of training should be to condition the dog to think that the appearance of strangers is GOOD.  We often do that by repeatedly feeding the dog a great treat when strangers appear, and when they disappear we stop feeding the dog instantly.  After a while, the dog likes it when strangers appear;-))  Also, if the dog will not eat, we know that we are too close to the stimulus (in this case, strange dogs or people), so we back up far enough so that the dog will eat, and we do the exercise, only gradually decreasing the distance between the dog and the stimulus.  Good reference books for you:
"Feisty Fido" by Patricia McConnell
"Scaredy Dog - Understanding and Rehabilitating Your Reactive Dog" by Ali Brown
They should both be available at dogwise.com or amazon.