A bed of nails can actually be comfortable, if there are enough nails. A person's weight is divided by the number of nails under them. Here, Mr. Slab weighs 200 pounds, so the default bed is actually extremely painful. Adding more nails will help him a lot. You can replicate the experiment with a nail, pen or pencil, and a few ounces of pressure somewhere on your skin.
Sidenote: most accidents occur while getting on or off the bed.