By just two weeks old, puppies are typically happily wrestling with their litter mates. While it might just look like fur balls having fun, this early play is critical to a puppy's social development. Dogs playing together from such a young age teaches them communication and self-control. After all, if you bite one of your litter mates too hard he won't play with you anymore.
As puppies age and grow, they don't lose their playful spirit. Give your dog the opportunity to make four-legged friends, but be vigilant. It's up to you to supervise and make sure your pup is enjoying good-natured play and isn't getting too aggressive with other dogs.
Here are a few signals dogs use to show other pups they're ready to play:

When does a line get crossed between play fighting and a dog actually exhibiting aggressive behavior?
Signs of dog aggression include raised hackles, stiffness, snapping, or lunging. If either dog shows aggression, separate them immediately. But be careful: Never get between two fighting dogs.
Dogs can also become territorial, whether it's toward a place, food, a toy or a person. If you notice one of your dogs becoming territorial over something any time another dog comes around it, it is best to separate them before aggressive behaviors begin. Then, work with an obedience trainer to try and get to the bottom of this behavior and train it out of him. This can often happen when introducing a new dog into the house with an older one. The older one is not used to having to share his toys or your love, so it might take a little extra training on your end to get him used to the idea of sharing his home.
If you have a dog that is aggressive, it simply might be best for you to avoid situations that may cause your dog to fight. But if you have had a dog that has shown signs of aggression in the past, you should be on alert for his whole life. A relapse can occur at any point. Consult your veterinarian if this behavior is regular. You might also need to consult a behavior trainer that can train your dog the right and wrong ways to act if you are struggling to train him to play nice.