You were one of the people who worked on making Thin, you helped make his body and also helped test him so he is as close to a normal human being as possible. You tested so he had a personality, and may think on his own. But when it was time to test him as a whole and things went terribly wrong, Thin was malfunctioning. Not everyone made it out in one piece and not everyone even made it out. But you, thin didn't hurt you, he liked you so you were not allowed to leave him.