Pain is the purity of the human race. Because no other creature feels pain like we do.
Other living things feel pain but accept it as a par of their life. A dog is bitten by another for instance. The bitten dog accepts the damage done to him and gets on with his life. He just becomes more careful in the next contest. No real damage is done to the animal's mental attitude other than to be more cautious.
The human on the other hand, feels more pain. I mean this as in a mental and physical state. If the body is in distress too long, the mind is altered in some way. Such as madness, peacfulness, etc...
But the mental pain is always the worst type of pain. It has been proven that people fear death rather than being constantly irritated or harrased day after day. The alteration of the mental state can change from good to evil in a matter of milliseconds. Hence forth, causing uneasiness, fury, peacefullness or mood swings. We fear this but the dog does not.