dog started showing signs of aggression

Question
Hi, I have a beagle mix with a chow chow dog. He's three years old. Today i walked him to the park, normally when we walk by a dog, he would try to sniff the dog and then walk away. Today, when the dog walked by, he charged at the dog and they both started barking at each other. That never happened before, he never barked at another dog. And when we were at the park, a guy in a hat and sunglasses try to pet him, he growled at him, which barely happens, if he doesn't like the person, he usually just shy away from that person, but this time he growled at him. I'm afraid something is causing this kind of problem. I don't know if this info helps, but my cousin just moved in to live with me and he kind of play with the dog roughly. Please help, I don't want my dog to turn into an aggressive dog.
Thanks

Answer
Your dog may have responded, by barking at the passing dog rather than greeting him normally, to a signal from the other dog that you didn't understand.  For instance: two dogs (A and B) approach, A may lick his lips, turn his head, approach on a curve or slow down (if not leashed), then A and B will stop, sniff each other, move in a circle while doing it, one might sit suddenly or play bow.  In your situation, A may have made direct eye contact with your dog (B) as he approached: being on leash, both dogs are forced to walk in straight line toward one another, making it impossible for natural body communication and aggravating any possibly dominant posture by A.  One small part of temperament testing any dog is to walk directly toward it without hesitation, making eye contact, while the dog is restrained on leash.  Most dogs will give calming signals when you do this and some will attempt to back away, will sit or lie down, while others may raise hackles or growl and come forward.  This may have been a SINGLE occurrence.  As for the guy in hat and sunglasses, both these things create of a human a strange appearing entity; sunglasses are particularly problematic because they hide the eyes and large hats can be intimidating to a dog.  Your cousin needs to keep his hands OFF YOUR DOG, rough play is NOT allowed, especially in this mix.  The Chow Chow is an especially difficult dog, not a casual companion, and requires positive reinforcement training and a strong, consistent leader.  DO NOT allow anyone to interact with this dog in any manner that provokes growling, using teeth, or strength, in any contest.  Tug of war is OUT and so is wrestling.  These are not games, they excite the dog and cause anxiety.

Go to the following site, which is Dr. Ian Dunbar's online YouTube video site
http://www.youtube.com/user/DogStarDaily

Look for "Meet, Greet, Repeat SIRIUS Adult Dog Training" by scrolling down in the right column, it's about the sixth choice from the top.  This is the ULTIMATE behavior you are aiming for which requires a lot of training: teaching your dog to follow you on "come along" signal, teaching your dog to "sit" on cue (command) everywhere (indoors first, then outdoors), then teaching your dog to "sit" on the street when another dog is passing and NOT interact with the other dog unless you say "OK" and the other dog's owner is able to control his or her dog in the same manner.  YOU must do the WORK by introducing positive reinforcement training in short, daily sessions and continuing to socialize your dog daily.  While you are preparing and teaching, as another dog approaches you on the street, go in reverse direction, get your dog's attention, turn back toward the other dog, and observe BOTH dogs closely.  If the other dog appears stiff legged, hackles raised, ears forward, go back in the opposite direction, engage your dog by making eye contact, then return in original direction (where the other dog will by then have passed.)  You can learn how to read dog body language, it's really quite simple, by going to Turid Rugaas' site:
http://www.canis.no/rugaas/index.php

You'll easily be able to see what your dog is communicating to an approaching dog, and vice versa, by observing them.  If your dog now presents with barking/lunging EVERY TIME, you'll have to do reverse direction quite often until you have a solid "SIT" outside, then reward your dog for maintaining the "sit" by giving him some treat and letting him see there's more where that came from.  The entire process involves desensitizing the dog to the approaching "threat", making the dog perceive this approaching object as rewarding, and letting the dog know you are in CONTROL.  As for people on the street, while it's nice for people to be kind to animals this does not involve getting into a strange dog's face, bending over him, forcing oneself on him: it's stupid, it's bad for the dog and it sets the dog up for a bite and you for a lawsuit.  If someone really, really wants to talk to your dog, have them ask him to "sit" and offer him a tiny treat.  You can arrange for several friends or relatives (people the dog already knows are ok) to work with you over the course of an hour or two.  Get four or five to casually meet you on your walk (one at a time over the course of an hour or so), you ask your dog to "SIT" as you stop to talk to these "strangers", then have the "stranger" talk calmly to the dog while you offer him a small treat (so long as the dog is calm and happy).  This might be all that's necessary to eradicate that one response to that guy with the hat and glasses.  

The YouTube DogStarDaily offers many fine videos on how to teach your dog basic manners: "sit", "come along" (or "heel"), etc., and you can easily learn by watching them and putting it into practice.  Dr. Dunbar also has several books all of which can be found on DogWise.com or Amazon.  You must also have your dog neutered, if he is not neutered already.  Any further questions regarding this work, please re-post using "followup" feature.