Pain is felt in the brain. The pain centers in the brain can get hype reactive and so can the nerves transmitting pain when the pain is long term ( or chronic). Chronic pain is unique in the sense that the pain you feel will be disproportionate to the level of damage or disease. For example in chronic back pain you may not even have serious back problems but yet the pain can be debilitating, disabling and deep. This also lowers the brains pain threshold. The more chronic the pain, the worse the brain suffers and this can also lead to irritability, low moods and lack of motivation -same as depression.
This is where antidepressants come in- as these work by increasing a natural neurotansmitter called serotonin. If you have normal to high serotonin levels it keeps you happy and makes the pain much more tolerable. This breaks the pain-mood cycle and releases natural endorphins that also help to move and exercise better. Eventually it helps to break the 'pain memory' and improves function. Chronic pain needs to be treated this way- using standard painkillers alone will never work and the patient may end up taking higher and higher doses and suffer more side effects if we do not use newer medications and more scientific approach such as using cymbalta.
There are many drugs that have multiple uses- some ant-depressants can be used to treat addiction, smoking withdrawal, bladder problems etc. etc. Hope this helps.