And some introverts don't get that when their friend who walks the edge of the introvert/extrovert line wants to spend time with them, it doesn't mean "we have to do something and be social". My most memorable high-quality interaction time with my best friend was the random day last November when he started playing a video game and I curled up next to him very quietly. It was interrupted with talking from time to time, but the bits I remember most clearly are the bits where we were just together, each alone with our own thoughts. And it was Safe.
I got used to our quiet mornings together at school. I resented it when others intruded. I got used to having that time that was not isolation but not interactive. "Companionable silence" is the traditional phrase.
I learned how to get him to talk to me. You do know how to get an upset introvert to talk, right? When you see they're upset, sit down a safe distance away and be very quiet.