I've worked with dogs who have had their growly drama beaten out of them. "Suppress" behaviour and you will be "creating a time bomb" as Dr. Ian Dunbar says.
I like Dr. Karen Overall's term "Information seeking". ie. If I do this...what will you do?" I call it being "opinionated".
It bugs me when people lump all displays into the "aggressive" term. Dogs use the tools they were born with to let you know they need your help. Majority of the time they've asked politely well before the drama. Some tv shows are NOT helping the matter.
My definition of "aggressive" is:
Intent to do physical harm. CONTACT and intent on drawing blood is where I believe the term "aggressive" may come into play.
My definition of 'opinionated":
In my eyes the problem starts when people don't realize they're ignoring all the non confrontational polite calming signals directed towards you. If you're doing something they're not comfortable with the majority of the time they'll let you know well in advance that it's not sitting well with them. Subtle signs like; the look away, lick lips, sniff, back away, yawn. If you don't take note at that time, the dog will kick things up a notch and use more overt agonistic signals like; freeze, hairy eyeball, growl, lunge, air snap, or even a pressure bite, barely felt by your skin. If they INHIBIT themselves from a full on bite, puncture and tear causing stitches...I call all those signals being "opinionated".
You're doing something that they're not comfortable with, for whatever reason, and they're providing you with feedback. Respect it.
Video courtesy of a foster Beagle Wags who used to be possessive of people and balls, watch him communicate with Daizy:
He went from a slight freeze right to lunge. NO NOISE.
Notice Daizy my Beagle's expression? She was like...'what was that all about', and looked up to me for information. She did the right thing by NOT taking things to the next level. She knew through our work, sometimes the best thing to do is diffuse a situation. I didn't yell or get physical, I just took note and we worked on creating a NEW association when people or other dogs were around his favourite toys or people.
It can be tricky if a dog has been punished NOT to communicate when they need your help reframing a situation to a more positive experience. Teach them HOW to use subtle body language and if need be, their voice before the lunge and contact has been at times a little hair raising.
Good news is once the lightbulb goes off in their head that someone is listening to the subtle signals, it's like a sigh of relief for the dog.