Pain isn't just a physical feeling, it has a lot to do with how we perceive it, and part of that is in social context. What this means is that we are used to seeing certain emotions or reactions that relate to pain, so when we do, we remember them because it warns us that certain things are going to cause pain.
For instance, if you just saw someone get into a hot tub and they made a face that indicated that the temperature was painfully hot, you would expect to feel the same pain when you got in. On the contrary, if they made a face that expressed happiness and relaxation, even if the water was the same temperature, you probably wouldn't think the water is painfully hot, but relaxing and enjoyable.
In this experiment participants are tested on how they react to the social context of pain by allowing other participants to trigger a shock or not, based on movements. If the test-participants felt safe with their movements they didn't get shocked.
Read the full story here :: Pain: It's not just personal