Smile a Little!

Have you noticed something strange about certain people (I don't have anyone specific in mind, just a certain class of people, of either sex, in case you were wondering... :D)? Smile at them, and all you get in response is a determined look in the other direction. This is often coupled with an expression that seems to be comparing you with the scum-at-the-bottom-of-the-pond, or worse, the-dirt-underfoot. I just don't get it. Why can't people smile back? It's not like their precious smile is a limited commodity, to be doled out sparingly, at best. And it's not as if I'm a walking freak-show, to be avoided like the bloody plague.
Come on, all I'm asking for is a bit of civil conduct, nothing more. I'm a human being, just like you. I don't expect anything from you. If I want something, I'll ask. Then you can look away and frown. But if I just smile at you, basic courtesy says that you smile back. I may not know you from Tom, Dick or Harry, (or the female equivalent, if you prefer) but I expect a certain level of civilisation from you. It's what separates you and me from the animals.
In case the above paragraphs seem to be written in anger, nothing could be further from the truth. It's more pity than anything else, to see that we've "advanced" to a stage where we don't seem to want to acknowledge a stranger's (sometimes it isn't even a stranger) presence. All we care about is ourselves, our small circle of friends, (if that at all) along with our petty lives. Sad.