There is not a single person on earth who did not experience a feeling called 'pain'. From the pain of getting out of the mother's womb to the pain of the final depart of the human soul; life is always cautious about pain. And the feeling of sympathy comes from the empathy of pain. Let us try to go a little more deeper into pain.
What modern physiology says is that, "pain is initiated by stimulation of nociceptors in the peripheral nervous system, or by damage to or malfunction of the peripheral or central nervous systems." That seems to be very logical, and scientifically agreeable. But let us try to understand pain more than what physiology says, by experiencing pain, by understanding its true meaning and finally by generalizing it .
Pain is always within us, yes, always! You can experiment it yourself. Just sit quite for a while, in a most comfortable position, in a most comfortable environment. And then observe, concentrate on each part of your body, your internal organs, your blood vessels, your brain, or anywhere, indeed you will sense pain from everywhere. In fact we are getting so much used to pain that we cease the feeling of the "pain of existence". You get accustomed to the condition you are in, which is often called homeostasis. A person having less pain than you may feel sympathetic towards you, on the other hand you may not necessarily feel the pain that the other person perceives in you. And instead you may feel sympathetic to someone who has more pain than you.
Now, since, pain is attached to all of us, very intimately and almost always, you may feel a bit perturbed while generalizing it. You may change the name 'Pain' to the name you wish, say an 'X factor'. But the nirvana of it shall be felt only when the word pain is maintained throughout. Hence I am sticking on to the word pain.
Our body can be called as an open system. Like every physical system, our body too tries to sustain itself. And when there is instability, our body experiences pain. While we generalize this concept, pain can be said as “an immediate consequence of entropy". So, when an animal's body is amputated it experiences pain, when a plant's shoot is cut it experiences pain, and when a mass of rock is broken into pieces it experiences pain!
In a world of Big-Bang, pain is inevitable. The birth of world itself was painful. Hope there will be a Big Crunch ahead where the opposite of pain is experienced, and if we are alive, we shall also be a part to experience that phenomenon.