To say that a dog is “aggressive” can mean a whole host of things.
 
Many times, pet parents don’t recognize the warning signs before a bite, so they perceive their dogs as suddenly flying off the handle. However, that’s rarely the case. It can be just milliseconds between a warning and a bite, but dogs rarely bite without giving some type of warning beforehand. 
Many of us who have been around a lot of dogs have learned to read almost by a sixth sence when trouble is brewing. 
 
 
 

 
This is a photo from our files of one of our  Timid Adopted Bostons.

 Before beginning any kind of behavior modification program take your dog to a veterinarian for a complete physical examination. To treat any kind of behavior problem without first ruling out a physical cause is grossly unfair to the dog. A perfect example is a Boston whose owner died and the family turned him into rescue. He was so "aggressive" towards people, He was attacking shoes and snapping a people. We decided to bring him to my home to assess if he could be adoptable at all. I thought at first it was just fear and confusion... after a couple of weeks and he was still unpredictable.  Due to some reading and talking with other Boston owners, I started noticing he was drinking more water than I thought was normal for a dog that had been with us for that length of time. We did a blood panel and duh, he had diabetes. We put him on insulin and he's disposition almost immediately changed in to an much easier older dog.  Due to the cost of insulin and the need to give twice daily shots, we decided that he was long term foster.  I just want to point out how treating a medical condition can sometimes turn around a problem dog.  RIP Diamond I still miss you... 

 

Sometimes dogs that are behaving aggressively, only because of confusion about their status and role in the family (pack).  This would describe one of the females in our home pack. Cally is getting much better, but she needs to be introduced slowly to others. I recently had my daughters three dogs visit for a month, I knew them since they come every year for "their holiday" of a month or so. I kept Cally behind a doggie door for about 24 hours watching how all the other dogs in her pack got along with the new dogs. She was the only one that did not know the others.  Then after giving all our guests, time to become relaxed with the home pack again. and the environment,  I turned one at a time in the yard then brought out Cally. I would then remove the guest dog put Cally back inside and in a little while the next dog.   This photo is the last of the three she was introduced to.. By this time the guest dogs smells were all over the yard and house.  Since none of the other dogs were upset (excited yes, upset no) at having them in the house and yard she decided not to be either.  But I still ensure she has time by here self in another room for a couple of days just in case. And never leave her alone with the other dogs she is starting to relax and so are the others beginning to totally ignore her... ( now I can relax a little too!)

Now really I would not call those cases "Aggression" to me more like failures to communicate.   Before you decide the dog is truly AGGRESSIVE have him examined by a veterinarian.. Everything from thyroid dysfunction to an abscessed tooth can cause aggressive behavior. Even if the dog isn't displaying new behavior if he hasn't had a complete physical exam in the last six months, do it.

If your dog is an unaltered male any treatment for aggression is going to be an uphill battle. Neutering will not solve the entire  Sometimes even spraying a female dog will improve her attitude problem.  If 90 percent of his problem is hormone overload, neutering will solve a lot of the problem.