Humans are social animals. Most will suffer greatly from social deprivation and eventually will start talking to oneself, get depressed, lonely, etc. That is when the stability of the mind begins to be questionable to oneself.
Unlike Oilersmyth wrote, there is evidence of this. Watch what happens to a man in the forest after being alone for over a month. You only need a couple of minutes to get my point: http://www.youtube.com/watch?v=8qIvGmVznWw
Unless You're a lone wolf, don't be alone for too long! : )