Our friend also told us not to tug of war with terriers b/c it will make them aggressive. but I disagree. Your dog knows when it's play growling (ears up, no teeth showing, tail wagging) and unhappy growling (the opposite). Uni loves to tug, she will tug tug tug and if you let go, she's like WTF! you let go!
She loves to fetch. She loves to have me watch her chew on something. If I leave she will come over and bark at me. So I ask her, what does she want and I follow her. She'll go back to her chew and start on it again. I leave, and same thing. So I've come to suspect she likes to me just look at her.
