I get a lot of new clients because their dog has growled at someone, maybe even a family member, and they are worried that means the dog is aggressive. Dogs who growl are actually trying to avoid using their teeth - the growl is a last ditch effort to get the human, or the other dog, or whatever, to stop doing whatever they are doing and move away. Here is some background info about why a dog might be fearful- it doesn't have to be a history of abuse.
If your dog is growling, there is a good chance you are missing a bunch of other signals before the growl. They may show in the blink of an eye that only another dog would see, too. Canine body language includes stress signals like an averted gaze, lip licking, sideways looks where you see the white of the eye, yawning, furrowed brow, shedding and so on. Some of my favorite resources about body language are linked here and here.
When you hear the growl, be calm and move away slowly if the growl is directed at you. If it's at something else, increase the distance between your dog and whatever triggered the growl. Never punish the growl by yelling, hitting, or jerking on the dog or we risk teaching the dog to go straight to a bite, skipping the growl.
A frequent comment I hear is that the growl was sudden and unexpected. That might be because the dog has been tolerating something for a long time and just can't, anymore. You are your dog's advocate and must not put him in situations that are scary and hope he can handle it. That's like taking a little kid afraid of water who can't swim and throwing him in a pool, then hoping he doesn't panic and drown.
Another reason a dog might growl about something he usually tolerates is because of something trainers call "trigger stacking." You know what this is, even if you are unfamiliar with the term. Think about a Very Bad Day. That is different for everyone, because different things, or triggers, make each of us feel stress. For me a Very Bad Day might be my older daughter oversleeping and missing her bus, so then I have to take her to a different bus stop and then I am not home to help my younger daughter catch her bus. Then I am running late so I miss my workout that keeps me sane, and I am frazzled so I leave without my treat pouch and have to circle back, making me late for an appointment, so now I run late for every appointment, all day, and miss lunch. Then, when my older daughter gets in the car and asks me to take her to Starbucks, I might snap at her. That snap would be baffling and upsetting to her because normally mom either says yes or no, and it's no big deal. Why am I snappy about a simple question? Trigger stacking! Things that stress me out have piled on top of each other all day are called triggers. They have stacked up and I am at my limit. For dogs we call that limit a "bite threshold" where they are willing to use their teeth if the triggers don't stop stacking. Snapping feels good in the instant because it relieves stress, although it feels bad later. Check out an infographic here to see what trigger stacking looks like.
Your dog feels this build up of stress, too, so a big focus of my training plans includes reducing stress. The first step is identifying as many triggers as possible, then deciding if we can adjust and manage the environment to get rid of any of them. That might mean closing blinds so he isn't barking out the window all day and turning on background noise (classical music ad audio books rank best in research studies). We might look at releasing stress with exercise and adding chew toys to his environment, as long as he isn't a dog who guards resources. It might mean a full medical work-up to identify any pain, itchiness, or discomfort the dog is feeling. For triggers we can't simply get rid of, we can help the dog dislike them less, or even learn to love them with a process called counter-conditioning and desensitization to make the triggers "smaller."
We can use anti-anxiety medication or supplements where warranted to effectively raise the upper limit where the dog is ready to growl or snap or bite, though they should be used in conjunction with a comprehensive plan as they are not magic on their own. A good example of a trigger that almost certainly requires medication is a thunderstorm. Finding a vet who really understands medication for behavior is necessary to avoid using drugs like acepromazine, which make the dog look calm but may not reduce anxiety, and may increase sound sensitivity, which makes the fear worse over time.
If your dog has growled, snapped, or bitten, please reach out and set up a consult so I can help you identify triggers and to help your dog cope.