For the introvert, introspection is a huge part of worship and spiritual growth—we can't survive without it. We find a lot of social interaction draining, and disruptive to the world of thought.
According to some studies, roughly one-third of the public is wired this way, and western culture, since the early 20th century, has been decidedly extroverted. It's an uncomfortable culture for your introverted friends. Understand: when an introvert suddenly disappears from a social gathering, it's not because they don't like you, it's because they need to.