A new study published in the Journal of Personality and Social Psychology finds that loneliness can easily spread through a social network:
[A] person's loneliness depended not just on his friend's loneliness but also on his friend's friend and his friend's friend's friend. Participants were 52 percent more likely to be lonely if a person to whom they were directly connected (one degree of separation) was lonely. For two degrees of separation, the number drops to 25 percent and 15 percent for three degrees. [...]
An odd look or phrasing by a friend that wouldn't even be noticed by a chipper person could be seen as an affront to the lonely, triggering a cycle of negative interactions that cause people to lose friends. The upshot: A lonely person is likely to lose touch with another person, who in turn gets cut off from others, and both end up on the fringes of a social group.