I’m introverted by nature, so spending time with groups of people tires me out pretty quickly. Introverted people typically need time to themselves to recharge their batteries, to be alone with their thoughts. They’re typically more reflective than most because they feed off that solitude.
I’m speculating, of course, because that’s merely been my experience as an introverted person, but I’ve heard from friends that they can be the same way (how do you think we became friends?). However, if you combine an introvert with a mental illness, that alone time becomes increasingly more complicated – and not easy to come by.