Have you ever heard of smile mask syndrome? If not, then you are not alone even though this is a very real psychological condition.
It turns out that it occurs when an individual develops both depression along with a physical illness because of a prolonged period of time where they have been forced into a completely unnatural smile. It was first noticed, or classified, in Japan back in 1983, and that in itself may very well be a reason why the syndrome exists to begin with.
In Japanese culture, there is almost a forced need to smile as much as possible so people effectively grin and bear it as much as they can. This leads to them becoming depressed as they do not want to do it but culture dictates it. So, while the outside is showing one thing the person on the inside is experiencing something completely different.