Why Does My Dog Eat Pillows & Sofa?

I'm wondering the same thing.  I have 2 dogs -- both mutts -- that are close in age.  The older one doesn't exhibit this destructive behavior but the younger one does!  Don't talk to me about treat balls, etc. Because he has all of this!!  They have a dog door to go in and out when they want and all the treats, balls, toys they could want.  However, the youngest one just destroyed the last back pillow on a fairly expensive sofa!  What can be done?