No, you should never assume anything. But I think that's beside the point. Whenever I see a beautiful smile, like yours in this pic, I don't care if it's fake or genuine. It makes ME feel good, bringing a smile to my face, and that's what matters most. So I'm either happy for the person for being happy, or else I admire that they manage to hide it so well. Of course there are all the unsuccessful attempts, which usually show, and no, they don't have the same effect on me. Still, in those cases I may give them an encouraging smile for at least trying, and hope that it will have a positive effect on them.

In the same way we should not assume that someone is in pain or angry just because they are screaming on top of their lungs. They may be just immensely happy, wanting to share it with the world. (Or in my part of the world they may want to sell something... ugh!) Well, this tends to have the opposite effect on me: usually it bothers me at indescribable proportions. So once again, the reason behind such show of emotions is secondary. What I consider more important is how it makes me feel. And I always prefer a fake smile to a genuinely happy scream.