St. Thomas Aquinas tells us that Human nature desires what is good and the absence of that good causes pain.

The origin of pain may begin in the body which has been injured or caused by viruses or germs living within our human body. The body by itself cannot feel pain. During surgery we do not experience pain due to partial anesthesia which prevents nerve impulses from reaching conscious level or total anesthesia which renders one unconscious or unaware of injury to our body. A dead body does not feel pain because the pain can only be experienced in one's soul. In order to have pain we must have the perception that something belonging to either body or soul has been taken away. A person who has lost his left hand feels phantom limb pain, that is heat, cramps and stabs. But, if the right hand was superimposed to the left hand by use of mirrors, the pain stopped. For how long is unknown. I presume as long as the mirror image can fool the brain. So, pain is primarily in the brain, not in the body.

The second type of pain has its origin in external events or the possibility of an external event (e.g. a mother's apprehension for her son going to war). If the son dies in the battlefield the mother experiences sorrow which may continue every time she thinks of her son's death in war. Sorrow is a kind of pain of the soul.

Teens especially experience pain of soul due to their unrealistic unfulfilled desires a follows:

Thomas cramps wrote that it was good for man or woman to have sorrows and adversities in this life because sorrows reminds us not to put trust in worldly things which are passing. Pain and sorrows remind us that our destiny is in God.

Some also experience psychosomatic pain because of the unity of body and soul. A chronic arthritic condition may cause depression of the soul. On the other hand, a person who worries a lot will develop stomach ulcers. There is also pain chosen for a greater good which we call a sacrifice.

2. Sin is Pain