Why does my dog eat underwear?

He's seeking your attention and he's attracted to your scent. To stop this before the dog injure himself swallowing foreign objects that aren't safe, keep underwear out of reach (including secured dirty laundry area with a lid)... and if necessary, set up booby traps with underwear sprayed with bitter apple, which should repulse the dog (I'd only do that if necessary after you secure the underwear).

Dogs are drawn to the odors of their human companions, and chewing brings out the smells the dog is interested in