Whats going on in the mind of a reactive and aggressive dog

I witnessed a woman trying to get her dog to listen as the bulldog lunged at the smaller shitzu. "Why doesn't my dog listen? I told him no." "What do I do?This is a common question when some people have a dog with behavioral issues(i.e Lunging at people or biting anyone who gets close. I usually tell people that a dog usually displays this behavior because in the past this got them out of an uncomfortable situation. This usually happened when a person or dog invaded their space and they were uncomfortable. The dog barked or lunged and they got their space back as the person or dog moved away. Aggressive dogs are rare. My definition of aggression is a dog with intent to hurt. Most dogs as i have said, want you to get out of their space not hurt you. Protection dogs have been trained to hurt and inflict a strong bite to the target. Its the difference between a street fighter and a trained boxer. The street fighter might hurt someone , but they really want the person to back down whereas the boxer is trained to fight and thrives in that realm. Its all about intent.  I plan to work with protection dogs somewhere down the line. It is a lot of training and can put a lot of stress on a dog. That why it's about genetics. Certain dogs have "it" other don't have "it".When I get a dog thats reactive i work on his reactivity directly by correcting reactivity. How do I do this? Most dog owners wait until their dog gets riled up to try and stop the lunging or excitement. As soon as I see a dog staring too long I correct. Its not about staring its about his intent. I also notice that a lot of dog owners give a weak correction. If the dog is still pulling it isn't hard enough. Its too hard if the dog screams. We don't want that so my suggestion is somewhere in the middle and allowing the dog to get comfortable with socializing. But don't stop there. Its also important to stop other dogs from invading the dog's space. This is the root of the problem anyway and I have always been in favor of the holistic approach to life. It is very important to take it slow. I will usually put a muzzle on the dog just for safety reasons. The dog didn't get this way overnight and it will certainly take a while to get over it. Everyday is different so it might take longer or shorter depending on your dog.