I've always read that playing rough with puppies will cause them to become agressive. Here is part of an article on it:
When you play with your puppy and adult dog, you can easily play too rough and in time you will produce a very dominating or an aggressive dog. Problems like food guarding, biting, nipping, and many other problems can be a result when you play with your dog or pup too roughly. Children need to learn not to play rough, pulling on body parts of the dog, or hitting the dog. There is a child conditioning method which will help to condition the dog to minor child behavior such as poking and screaming around the dog. (Click here for the child condition method.) If you play too rough with a submissive dog, the dog may become fearful of you. (Click here for more about fearful dogs.) If you have a pup or dog which is nipping or biting click here. (I'm still working on the fearful and nipping or biting pages.)
You can read the rest of the article here:
PLAYING WITH YOUR PUPPY OR ADULT DOG
Michele :-)