It is essential to understand how and why pain occurs if we want to address it effectively.
Here is a very basic example to illustrate the main concepts. Something touches your foot. Nerves in your foot send a signal to the brain. Once the brain receives the signal, it interprets the type of sensation (pressure, soft, sharp, hot, cold, itching, stinging, burning, etc.), assesses the intensity of the sensation (light vs heavy, brief vs prolonged), and determines whether the sensation is bothersome.
Once the brain has categorized the sensation, it can decide how to respond. As you can imagine, the appropriate response differs based on whether you just stubbed your toe or you are slipping your foot into a warm bubble bath. The brain then sends a response signal back to your foot via a different set of nerves, making you jump away from a sharp nail or boiling water, for example.
Our brain also interprets sensation based on situation context. If you step on a sharp toy that your child left in the hallway (AGAIN!), the combination of feeling both pain and frustration can amplify the discomfort you experience.
The brain also has a remarkable ability to triage sensations - in other words, to decide that a particular sensation is important and get you to pay attention, or to decide that something is not dangerous and allow you to ignore it. For example, when you are sleeping, your brain is constantly receiving signals and deciding whether is it safe to stay asleep. When you are at home sleeping safely in your own bed, your brain usually interprets routine information, such as a car driving by or your roommate flushing a toilet, as non-threatening and therefore you keep sleeping. Contrast that to the experience of staying at a roadside motel that looks like something out of a scary movie. Every time a car door slams or window rattles, your brain interprets this information as potentially threatening and you jolt awake.
The system by which we feel, interpret, and respond to pain works fairly well for most acute pain episodes. Unfortunately, chronic pain is much more complex.
Chronic pain can result from dysfunction at one or multiple steps in the system. This could include continued pain signals from a chronic tissue injury, abnormal signaling from a nerve despite lack of ongoing tissue injury, or abnormal triaging mechanisms that amplify sensation.