Help! Dog is chewing fur off of his back?

The past couple of weeks, my dog has been chewing on this spot on his lower back. He keeps chewing and chewing until he has to stop to breathe. He's chewed all the fur off of the area. He's a furry Samoyed, though, so I just brush over the area. What could be the problem? And is it something to even worry about? I've checked him for fleas and ticks, and didn't see any.