You may find it interesting watch on the of the collie and the terrier. Great interaction between these two dogs.
Watch the fantastic body language, you'll see a mix of appeasement gestures and a couple of "that's mine" ones as well.
Other videos show goings on at Good Dog Training School. My dog training venture. See what we do in a socialisation class and how to decide if your dog is a "bally" dog and will happily play with them. If a dog is interested in balls you can use them as rewards for training, you may even find your dog enjoys activities such as flyball or flygility. One word of warning about balls. Don't spend hours throwing them for your dog. This will get his adrenaline levels really high which can lead to stress and he'll find it harder to settle down. Rasied adrenaline levels can also lead to agression as the dog can loose self control. If you dog happens to get an injury while playing he'll be less likely to notice as adrenaline inhibitss pain. 3 or four fetches are enough for a play session.