Why do horses roll in dirt?

Sorry, but this is going to be a pretty doggone long answer. Instinct, my dear. Wolves, in the wild, would roll around in the grass - or poop of the animal or creature they are hunting - to partially cover their scent, making it easier for them to approach the prey that they are after, and for them to get closer to them or it before it or them notices. True, dogs today don't have to do this, but the bloodlines of their dear cousins still run through their veins, and this cannot really be stopped, other than training. But generally, it cannot be stopped. Another reason is to itch a hard to reach spot, one that the doggie is just dying to have scratched. Another logical answer would be because it feels good. The good green grass is usually, for the most part, cool and slick. Another concluding answer is because they are expressing their joy, happiness, and playfulness.