It does not matter what kind of dog you might own, whether it's a pit bull, a poodle, a maltese or whatever, all dog breeds can become aggressive. Aggressiveness does not mean that your dog is a bad dog because there are a lot of different reasons why your dog might become aggressive.
There are a variety of different reasons why your dog might be showing signs of aggression. They might be feeling threatened if someone tries to challenge their social status, they might be afraid of something and become territorial or they may think they are protecting you.
You need to learn what the threshold of aggression is for your dog. A lot of dogs tend to be able to put up with just about anything without even letting out a simple snarl. Whereas other dogs might show that they have the shortest of fuses and are even quick to snap at you or bite you. You need to figure this out before a severe event of aggression actually happens.
It's always a very good idea to make sure that your dog is neutered or spayed. Unless he or she is specifically going to be used for breeding. Otherwise there is no reason not to have this done to your dog. By reducing the hormones in a dogs body, you are also going to be lowering their desire to be as aggressive.
If you notice that your dog gets a little irritated at the vets or at the groomers then it's a good idea to get a muzzle for those times. This will be quite helpful in cutting down on the possibility of your dog getting upset and biting someone.
If you find that your dog has issues that you don't think that you can help him with it's worth consulting a specialist in animal behavior. These are professional people that are experts in knowing how to utilize animal behavior with modification techniques and this can help your dog to stop responding to certain things with aggression.
If you dog seems to get overly aggressive when you are playing around with him or playing tug of war and he tries to become the dominant one in activities of this sort try changing the way in which the two of you play together. Figure out ways to play that are calmer and gentler in order to avoid this type of dominant behavior in your dog.
Make sure that you go see your vet and see if there might be some medical issue that is causing your poor dog to be aggressive for what seems to be no reason. This is especially true if your dog has always been calm and suddenly has gotten aggressive. It may turn out that your dog has had a bad tooth and the poor thing has been in pain the whole time and had no other way of showing it except through aggression.