Just as we tend to do when we are afraid that situation is going to come about that is terrifying, a dog will often try to control their surroundings. This can show up as aggression of any kind, whether it be towards other dogs or towards people.
If you can convince your dogs that they can trust you to be in control of situations while at the same time changing their perspective of the world, you can help build him/her up into the calm, confident dog it wants to be.