Why do dogs do what they do?

So... why do dogs do what they do? Why do they turn three turns before laying down? Why do they cock their heads when you talk to them? Why do they eat random things?

My dog has a thing about eating grass... yet it's not like he has a belly ache or is missing on some nutrients... he just thinks he's a horse. He'll go out and munch on a few tall blades of grass here and there while he's out... dogs are silly.

My dog has nothing on a friend's dog... now this dog should have been named Hoover. Can you guess why? The thing about it is he doesn't eat just anything... he eats only undies (clean ones)... what? My friend was telling me the story of his latest snack. She was taking the laundry out of the washer and into the dryer when she dropped a pair of undies and before she could react Hoover was out the door with his snack. He isn't like dogs that will chew on your socks or item till it's all in pieces... he devours it immediately. By the time she got a hold of him the prize was gone. So far she's had to retrieve two pairs from his throat... she was able to grab just a piece of the material and pull them back out. A few have been swallowed only to be found a couple days later... and now she will play the waiting game for this latest snack to return. Why does Hoover do this? He eats it and then for the next two to three days he really can't eat anything or much of anything till he throws it up and can go back to eating his dog food... till the next time he gets that quick chance at a snack. Dogs are funny!