Noise in the wires
Well, maybe not. I happen to be shut-in, but for the last few weeks i had to go out every few days to do things that aren't "buying food really fast with minimum communication or interaction with people".
I realize that for most people going outside is a part of daily activity, but to me any sort of prolonged exposure is frustrating as hell.
Worst thing about going outside is not people (not that I feel comfortable around humans, but i can manage), not wide open spaces, noises or anything else, but the fact that I end up alone being all alone with my thoughts.
Under normal conditions I can usually deal with the clusterfuck of thoughts in my head (or just distract myself with easily accessible books/movies/music), but outside my mind starts to eat itself pretty fast. Outside brings out worst kind of thoughts and requires a lot of pressure to keep myself from falling apart.
2-3 hours of the outer world drain me of energy for the rest of the day.

other stuffs