All horse behavior relates to search of pleasure and avoidance of pain.
Satisfying nature's instinct is more pleasurable than physical pleasure. Correspondingly, the stress invoked by frustrating these instincts can often be more of a concern to the horse than physical pain. A simple example illustrates this point. A new horse introduced to an established herd will often be driven away by the dominant horse in the herd. However, the new horse will keep returning to the herd, regardless of how many times it is kicked, bitten, etc. by the dominant horse.