Pain an experience when tissue is damaged or has the potential to be damaged.
Tissue damage is detected by receptors in the body that send information up to the brain. It is more than just a physical sensation, pain is perceived in the brain and will produce an emotional response. It is not just an on/off switch but is more like a dimmer switch.
Due to the ability for the brain to modify the pain signal, and each individuals unique response to this information the same condition can be experienced differently by an individual. We need to assess the animals response to the stimuli and treat accordingly rather than having a set expectation of how much pain is being experienced.
While most pain is useful there are other forms that are not beneficial which are called maladaptive or neuropathic pain.