First off, I want you to know that I admire, and often envy, your ability to instantly make friends, shine in the spotlight, and make decisions on the spot. You're a pretty great person, actually, and I'm glad to have you in my life.
But I believe your knowledge regarding the inner workings of an introvert's mind is sorely lacking. This is not entirely your fault--you've been raised in an extroverted world that values being heard over being introspective, being friendly over being shy.
It is not my intention to lecture you on a centuries-long prejudice. But perhaps if you give me a few minutes of your time I can help you understand a significant portion of the human race a little better, and then maybe we'll be able to talk about something more interesting than my nonverbal habits.
An introvert is someone who re-energizes by being alone or meditating on various life happenings. Trust me, there's no need to look so concerned when I tell you that I spent my Friday night cuddling with a book instead of a human being. I may accept an invitation to a party every now and then, but if I decline please don't take offense. I often prefer my own company over that of others, but it's nothing personal. And please don't worry about my well-being the day after a party; I may be exhausted, but I'll recover. Eventually.
Introverts are often erroneously pegged as moody or depressed, probably because of the expression created by relaxed facial muscles. This is one of those situations where you shouldn't judge a book by its cover. Facial expressions are unreliable personality meters, regardless of your genetic wiring.
And finally, introverts are silent because they preserve their vocal chords for opportune moments when they have something useful to say. They are not judging you, they do not think they are better than you, and they are not smarter than you (though you have our permission to assume this anyway). I should warn you, though, that stupid questions will be answered with scowls, glares, and excessive eye rolling. "Why don't you talk?" and "Have you ever had a conversation with anyone?" qualify as stupid questions. I, for one, have decided to stop trying to politely answer idiotic questions like these, and wholeheartedly believe that you deserve whatever damage is caused by my death glare.
As a common courtesy, I don't try to make you feel like a freak whenever you project yourself on everyone around you. I would appreciate the same treatment from you whenever I draw inward.