Sadly, puppy aggression can happen.
We like to think that our wee ball of cuteness just can't actually be being aggressive - it's just play, he's only nipping, she'll grow out of it.
Well, puppys do examine the world with their mouths, they bite their litter mates and other dogs or puppies in play, and soon realise to inhibit the bite. They may also just be having fun and not realising that bits of people shouldn't be in their mouths.
However, sometimes it is more than that. Puppies who have had an aggressive parent are probably more likely to be aggressive themselves - so it's important to assure yourself before you get your puppy that the parents have a good manner.
Both nature and nurture can create an aggressive behaviour. Commonly aggressive behaviour can result from fear and from over excitement that creates frustration and follows through to aggression, or they may be pro-actively possessing and protecting an object - it's "mine"!
The thing is, puppy aggression isn't something that they'll just grow out of, and given that you've probably just spend a few hundred pounds on your puppy, really the best advice here has to be to get help from a qualified behaviourist or trainer - one that will help you to understand the root cause of the puppy aggression, and what steps you'll need to go through to resolve the aggression.
Here are some quick hints and tips on puppy aggression, but please do speak to your vet to ask for help.
Puppies need to be carefully and safely exposed to the many things in their lives that they might have to deal with. If not, then they may become scared. Read more on puppy socialisation here.
Frequently, puppies who've spent their early weeks in a byre, or a quiet house and haven't been exposed to lots of noises, sights, sounds and people are more likly to be scared than those that those that spend their early days in a busy house. So, its good to consider this aspect when you go to see your puppy too. Using a desensitisation or socialisation CD can help to safely expose your puppy to noises before they can get out and about. Have a look at the CLIX desensitisation CD - it's good for dogs or puppies that are already scared, but also will help to prevent the fear arising.
Here are the most popular dog training help topics:
Help - Before Getting A Puppy
Help for - When Puppy's Just Come Home
Puppy Training Help Notes