Why do horses roll?

My Christmas gift has been glorious weather and hours to go riding. With the intensity of the season over and just a few final sweepings on my list, I've indulged. The things we've forgotten are huge. I had to start all over with the "drop head" bit. This is where you pull on the reins with a steady pressure and reward the tiniest drop of head.

I got out of the habit of doing this and it showed today. We stood there, facing the setting sun, Canyon bracing against his halter, me bracing against the saddle, reins taut. Around us horses were going through their paces, trotting, cantering. Dust rising from the arena, we've been without rain for so long everything seems to have a layer of dust.

Including our manuevers, apparently.

I'm waiting for the slightest give, which seems to come after a few hours. Of course it wasn't hours, but it was a very long time, minutes at the least.

By the end of our session he was dropping his head like an old man with narcolpsy.

We rode in circles, canter, trot, walk, trot. Basics. Side passes were elusive, although there were two. By the end of it all, I had one sweaty horse.

So, given that it was warm, I opted for a bath. (Next time I'm trying this instead) I use the one pictured above. Spray on foam. Rinse off foam. Spray on foam and leave it on.

Unfortunately I did't have the time to hang out until he dried out. So out we go to the pasture, stall by the water (where he proved the old adage, you can lead a horse to water, but you can't make him drink even though he was pestering you for the hose earlier, but since it had soap you couldn't give him any...), stroll him toward the hay which he IGNORES (even though he was loathe to leave it two hours earlier) and he walks with me until I "mentally release him" if you know what I mean. Then he spots a nice patch of dirt.

He circles this bare patch of ground, testing it with his clean hoof. He drops it his head low, ensuring that yes, this is that spot, doing a darn impressive dog imitation, then drops gently to his knees and proceeds to do a full body roll.

No, one side just won't do in these situations. After all he was GLEAMING. You gotta get that shine off all sides of the hide...

He rubbed into the dirt so hard I thought he was going to dig a hole and I'd find him later, hooves in the air, completely stuck.

The only thing left even slightly clean were his knees. Odd little white spots on a completely mud covered equine.

Is it so awful to be clean? Why do horses prefer being covered in mud?

I read here that a rolling horse is a healthy horse. Rolling can correct back problems, soothe muscles, keep flies away (did he not note the FLY SPRAY?).

Ah well. Next time I'll carry my camera for a before and after. At least I know he's clean under all that mud.