That's the thing, humans react to the stimuli and then feel pain. The reaction is subconscious. What if that's all 'pain' was? Subconscious and automatic withdrawal from a source of stimuli? That's not really pain as we conceive of it, and that brings on the question of whether or not it deserves the same moral consideration? If that's all certain animals experience, lets say bugs, then should we consider that tantamount to the consciously felt and misery inducing pain humans have?