There is a miserable sourpuss Defence Force?
Why does someone not wanting to change their facial expression at the whim of a stranger mean that they're a miserable sourpuss? What if they're happy and just don't feel like smiling just for the pleasure of someone they don't know? And what if they have very good reason for not being happy, like suffering from a terrible depression or just having a pet, friend, or relative die? Just being told to smile by a complete stranger is hardly going to make such a person feel better. Instead of telling a stranger to smile just because you say so, how about trying to do something to actually make them feel like smiling, like just helping them with something or actually trying to make conversation with them?
A person's hardly a miserable sourpuss for simply not smiling on the whim of another person pleasure though, with that person just telling them to do so without actually giving them any reason to feel like it--just try and imagine that from the other person's perspective, and imagine how they feel being told to smile by a complete stranger who has no idea what their current mood actually is, what's going on in their life or anything, and just wants them to smile anyway for their own benefit without knowing any of that stuff, just so they, a complete stranger who's unaware of what's going on with you or anything, can feel a bit better, without even knowing if there's anything wrong or if they can help or anything and just expecting the other person to smile on their whim anyway?
No offense may be meant by it at all and some people may say it with the best of intentions, but when you actually take the time to think about it, it's really not much of a kind or caring thing to say after all and is really just kinda stupid. If you want someone to smile, try and do what you can to make them happy or help them out or whatever, but don't expect them to smile or change their mood or whatever just because a complete stranger said so--that doesn't really make any sense at all and is really kinda rude.