A: To be comfortable -- and possibly because their instincts tell them to.
If you own a dog, chances are you've seen the way they walk in tight circles, stomp with their paws, or claw and dig before lying down. Depending on the dog and the situation, circling behavior can be cute (or, if excessive, can be annoying -- especially if you're trying to sleep).
So, why do they do it?
Part of the answer is obvious-after putting in a long day of work at a demanding, high stress job, dogs like to kick back and relax just like the rest of us. Stamping or clawing at a blanket may fluff up or beat down the material in a way your dog enjoys. (It's weird to think of an animal that eats its own excrement from time to time as "discerning," but any dog owner will tell you they're secretly connoisseurs of comfort.)
But that can't be the only reason dogs spin, since you'll also see them going through the same motions on non-fluffable couches and carpets. And even when they're not clawing, they'll often give a quick, cursory spin before curling up. Why?