Smile?

There are times and places where it is normal to tell someone to smile. When you are having your picture taken, when you are performing, when you are having your teeth whitened (I dunno...)
But for some reason, people are fixated on telling ME to smile when there is no call for it. I smile when I am amused, I smile when I am happy, I smile when I am having my picture taken, I smile at customers, you get the idea. I am not one of those people who never smile.
Now I get to the point. Random strangers walk by me, stop, look at me, and tell me to smile. It happens a lot! I'll be at the fucking train station reading a magazine and someone will say "Come on! Smile!" and wonder why I give them suspicious looks.
I don't know about you, but I don't particularly care to walk around all day like this

No..that's just silly. So why is it such a sin for me to have a straight face when clearly, nothing amusing is going on!!!! So usually I will give them a smile like this

and then they leave me the fuck alone.