Everytime I take my dog out and I see other dogs, I always check to see if they are aggressive. It is a good idea to not only socialize your dog with people, but also with other dogs. Dogs love to play with each other, but unfortunately some dogs have never been taught how to behave. For you new puppy owners, if you start early, (you should be getting your puppy at least 8 weeks old), your puppy will grow up to be a friendly adult dog and will not become aggressive with others. Take the puppy around as many different people and dogs as you can. Keep it away from the aggressive ones and always ask first. If you have a dog that is aggressive, now would be a good time to start thinking about an obedience class. Make sure you have a competent instructor. Go on-line and look for an obedience and/or agility club. A well-behaved, obedient dog will make the owner proud and possibly same you money in the long term.