People who do regular volunteer work tend to live longer, on average, than people who do not. It has been found that doing good,  a category that certainly includes volunteer work, releases endorphins, the brain's natural opiates, which induce in people a feeling of well-being. Clearly, there is a connection: Regular releases of endorphins must in some way help to extend people's lives.
The statement "People tend not to become involved in regular volunteer work unless they are healthy and energetic to begin with" most seriously undermines the force of the evidence given as support for the hypothesis that endorphins promote longevity.