Why do poses look unnatural?

I'm often asked, why do people lose their ability to look natural when they are getting their picture taken? Why do we suddenly become unable to stand or sit naturally? Why do we not know what to do with our hands? I'm reminded of that funny scene from Talladega Nights:

Why do we lose the ability to function normally? Why do portraits often look like this:

Did these people suddenly forget how to sit comfortably? Do they suddenly not know how to relax? Are they going through life looking awkward and weird?

Here's the secret:

Their subconscious mind knows how to pose their body naturally, their conscious mind does not.