The answer is behind door number two: younger less-trusting cohorts are replacing older more-trusting ones. The percentage of 18-to-29-year-olds who think most people can be trusted has fallen over the years and was just 18 percent in 2010.
Within cohorts, the percentage who trust others has remained relatively stable across the decades. One cohort in particular is consistently the most trusting--people born between 1941 and 1950 (aged 60 to 69 in 2010). The 51 percent majority of today's 60-to-69-year-olds think most people can be trusted.
The 33 percentage point gap in trust between 18-to-29-year-olds (18 percent) and 60-69-year-olds (51 percent) seems like a profound statement about something.