Hmmm... I think saying "Do not smile" is a little harsh. Though I know exactly what he's getting at, and I know he was trying to be funny.
Korean's definitely have some social rules about smiling, but it's fairly specific. I'm not sure about this, and feel free to correct me, but I think smiling, in Korean culture, is seen as showing a weakness. I assumed it was an old Confucian ideal that has stuck around like so many others, AND I think it's more prevalent with the older generations.
I know official pictures--school, visas, work--you are not expected to smile. In fact, when I went to get a visa picture taken, the photographer told me to smile, so I flashed a big one. He said, "No! Mouth closed!" The picture ended up being of me looking like I'm trying not to laugh. Also, anytime you want to show power in a situation, you probably wouldn't smile. It's all very Korean though. If that's the "way it is," that is what the ENTIRE society will do, and expect nothing different.
That being said, Koreans love to smile!! They're a warm culture who love to laugh. WHAT exactly they find funny boggles my mind sometimes--certain Korean television shows--I just don't get them. I love to make my students laugh, and I think they like it when I do. In general though, if you flash a smile walking down the street, there's a 50/50 chance it will be returned. Depends who you're smiling at and why. In my experience though, where ever you are, a genuine smile = a smile in return--and that still holds true in Korea. Come on, they're human too.
The Dokdo thing IS really serious here! It's funny to outsiders, but Korean's feel VERY strongly for the little pile of Rock's in the East Sea. Japan and China have been trying to take things from them for centuries...can't blame them for wanting to hold on tight. They still have their little peninsula--and they've fought for it. Maybe they think giving Japan an inch, will let them think they can take a mile...it wasn't that long ago that Japan occupied Korea.
I would like to hear though of any other theories on the whole, "no smiling" thing. Remnant of Confucianism, or is it something different? Anyone know?