my Yorkie challenges me!
 

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!

Well hopefully this will be Mister's last home. I think he is challenging you... trying to see if you are going to get rid of him aswell. Have you tried obedience? Some pups are very strong willed and maybe he would benefit from some professional help. He probably needs a lot of structure and needs to know that YOU are in charge. It can be hard to become alpha dog (it's taken me a while :p ).. you should try reading Caesar Milan's book Becoming the Pack leader. It's great and it sounds like it is something that Mister needs! good luck. hang in there.. it's hard being his 3rd home... he is confused and doesn't know where he stands in the scheme of things.