[sheshark]

My year and a half old Yorkie, Mister, challenges me, most times out of the blue. He stands alert and barks at me, kicks his feet and growls. We also have a labrador, Shadow, and they're good friends. They play well together, although at times Mister can become too agressive and they have to be told to stop. Mister will challenge Shadow for play, also, and can be persistent when she's not interested, but not agressively so.
Mister mostly challenges me when i've told him "No" about something ("no barking", "stay down" (out of the lap), "no chasing the cat"), when its feeding time, he needs to go out, or just for attention sometimes. Its been difficult with him to tell the difference between "i need to go out" and "i want attention" though at feeding and potty times he sometimes will submissive-roll and continue barking and growling. He seems very strong willed, taking his time responding to commands.
We are Mister's third home, he's lived with us about a month and a half. We've had Shadow since she was a puppy and she is well trained. Mister has left his other homes because the people we unable to devote enough time to his training, this may be an attitude issue.
Any advice would be welcome!