I appreciate that you wish for me to appear happier. But really, if you don't know me, it is quite a risk to suggest to me that it can't be that bad or to order me to smile. I imagine it makes you feel as though you are spreading good cheer to tell me to be happy. And while I recognize happiness is often a choice, you - since you don't know me and are not making any attempt to get to know me - just know that at this moment I appear unhappy. There are a number of legitimate reasons ranging from large to small that I might not be smiling, and none of them are improved upon by people telling me to cheer up. (I really think "cheer up" is a bit like "calm down" it hardly ever works on the people who really need to hear it, in fact you risk sending them farther in the other direction.) So, I am sorry that you did not see me in a moment of outward happiness. But, your words, however well meaning, are not helping.
Just Let Me Keep Walking and I'll be Out of Your Sight