> There's a reason people with less social contacts have, in general, a shorter life span

Or people who are obviously going to die earlier will get less social contacts -- the causation could be in either direction. Perhaps they don't form the social contacts because they don't want to make others who get to know them be the ones to suffer sadness when they die first, or they don't want to burden others with feeling they should look after them when they get sick first. Or perhaps it's the others who don't want to risk the sadness or incur the burden.