I do it to feel safe, and because sometimes I can't deal with any more stimulus or input. I only feel comfortable going out or being around people if I am coping in isolation without it taking everything I have. If I am struggling to hang on in private, there is no way I can deal with the minefield that is social or public situations. I have always preferred a great deal of alone time though, even before my anxiety was this bad. I guess that just comes with being an introvert, and being shy.