Letting out an aggressive growl is sometimes your pup's way of saying he really doesn't like something, whether that something is a cute bunny hopping outside the window or you walking past his food. Sometimes he just wants his big canine voice to be heard and his growls aren't aggressive at all. In other cases, he means business, and that's not allowed.
Aggressive vs. Playful
Growling is scary behavior that you should never tolerate. Well, almost never. Sometimes growling isn't scary at all and is no indication of aggression. It's simply your pup's way of being vocal while having fun. A playful growl will usually sound somewhat higher pitched, and often lasts for a couple of seconds. Your playful pal may sometimes sound like he's mooing. He might show his teeth, but often because a toy is pushing up his lips. Aggressive growls happen suddenly, seemingly out of nowhere. They're often sharper and deeper than a playful growl, sometimes combined with a bark, and become deeper the more aggressive your dog becomes. Your pup will typically give a big showing of teeth, although fearful dogs may keep their mouths somewhat closed. He might raise his hackles and his body will appear stiff and eyes still. Dogs who growl out of fear may cower. If your pup is playing with another dog, he might sound aggressive, but his hackles shouldn't be raised, and his body shouldn't appear stiff.
Lots of things -- in fact, almost everything -- can cause your pup to lash out with an aggressive growl. For brevity's sake, the most common types of aggression can be broken down into the following categories: possession, fear, pain, frustration, predatory and protective. As an example, let's say you walk by your pup while he's eating. If he's possessive over his food, he may growl at you. He thinks you're going to take it away. If he's not been socialized and he sees another dog while you two are taking a stroll, he may freeze up and growl out of fear.
The best way to deal with aggressive growling is to prevent it in the first place. If you're raising a puppy, mess with his food when he's a youngster. Put your hand in it, pick it up and walk near him. Play with him when he's chewing his favorite toys. Take him for walks and car rides so he sees and experiences everything. Sometimes aggression stems from your little guy's genes, but the way in which you raise him as a puppy can prevent a lot of problems in the future.
If you're already dealing with an aggressive growler, don't try to correct the behavior on your own. Aggression, especially fear aggression, is extremely difficult to counter condition. It's not impossible, but save yourself the agony and the potential for a nasty bite by contacting a qualified trainer or your vet.