Why does my puppy constantly bark?

I have an 11 month old cairn terrier. He barks every night for hours at his front paws. I'll take him for walks and play with him pretty much from the time I get home from work to the time I go to bed (about 5 hours). In the middle of playing fetch or tug or anything, he'll stop and walk away from me and start barking. If I'm watching TV or on the phone, he'll start barking too. And the only time he stops is if I get up and walk over to him OR if I get up and walk out of the room. I've tried everything - ignoring him, positive reinforcement when he's being good, negative reinforcement when he's being bad, and I've read/watched as much as I can from the Dog Whisperer. Why does he do this? How can I get him to stop?