Why Do People Get The Bends When Diving?

It has to do with the change in pressure on the body.  The Bends happen when the body experiences a quick decrease in pressure.  We mostly associate The Bends with a diver ascending too quickly from a deep depth back to the surface.  But "decompression sickness", as it is more formally called, can happen from changes in altitude, too, or even after exiting mines and going back to the surface.

According to Henry's Law, if the pressure of a gas over a liquid (our bodies are mostly liquid) is decreased, then the amount of gas dissolved in the liquid decreases.  Conversely, when one enters a more pressurised space, natural processes mean that more gas gets dissolved into the liquid (our mostly liquid bodies).  But when we return to a low pressure environment, the gas dissolves out.  

The gas in question is nitrogen, which is normally suffused into many of our bodily tissues.  If it bubbles out too quickly following decompression, it produces a range of symptoms, from joint pain to paralysis and even death.