I think whenever I look happy (smiling and groomed nicely) people want to know how I am doing.

I think people know intuitively how a person is doing for the most part but especially knows when someone is doing not so well. It's during those times when most people do not care to hear about woes so will ask a superficial "how are you" very quickly just as an intro to small talk. Somehow we are supposed to know that they expect to only hear a short response like "fine" or "wonderful" or "I'm doing very well, thank you."

I've often replied back "how are you" to their "how are you" and let them talk about their problems. I don't think they noticed I never answered their question.

The military is really bad about this. Military members are taught customs and courtesies about greeting one another. Like robots they'll greet each other, sometimes after a hand salute, and start talking business.

It's ironic that asking "how are you" has become the polite thing to say to people but it is impolite not to listen or care (if asking the question "how are you")