"“I missed the world in general. Seeing things move, seeing cars, dogs, the sun. My colleagues were amazing, and I couldn’t have picked better people to be locked up with, but you start missing meeting new people on nights out, the social variety,” he said. “For me, that was the trickiest part.”"
This seems to be a recurring theme in isolation experiments and have shown up in movies recently. In the movie, "I Am Legend" its clear Will Smith attempts to keep himself sane by using his imagination to create some kind of human interaction. I found it to be a profound theme running through the movie.
I think above all, this is a primary factor with the human condition. It's the need for social interaction. Without it, we don't seem to do very well.