What is pain?

Pain is perhaps, a reaction to the self, more than a reaction to stimuli. It is we, who respond to the stimuli. Stimuli is a series of stimulus. What is stimulus? Stimulus is any reaction, to phenomenon. What causes us to react in a dual nature to stimulus? We either react positively, or negatively to stimulus. Why do, we, more importantly, react sometimes, in extremes, to stimulus? All stimulus, should be a neutral experience. Then, why do we see the positive and negative to stimulus? We may perceive the same stimulus as broadly positive at one time, and broadly negative at another time. What is the change perceived in the stimulus, so that we can feel the stimulus as positive, and the same stimulus as negative? Is not the difference in our own reaction? What is an intense stimulus, and what is a moderate stimulus? When we perceive warmth, we respond moderately, and when we perceive intense heat, we respond appropriately. What is this appropriateness? Is not the cause of our pain, ourselves?