How can I let the introvert in my life know that I support him and respect his choice? First, recognize that it's not a choice. It's not a lifestyle. It's an orientation.
Second, when you see an introvert lost in thought, don't say "What's the matter?" or "Are you all right?"
Third, don't say anything else, either.
Check out the rest, if you are an introvert. If you're not an introvert, read it and learn!
Found via Marko.