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You might meet someone some day who is cheerful and friendly -- and patently insincere in his good-naturedness. Let's call him Melvin.
The way that you respond to Melvin's mannerisms depends on the way that you interpret them, and there are different ways to interpret them.
People with inclinations toward malevolence, will recognize Melvin's deliberate efforts at cheer, and instinctively interpret them as duplicitous, untrustworthy, and of course, malevolent. They will hate Melvin.
Luckily, the world is not comprised entirely of malevolent people, and some of us will realize that Melvin may just simply being kind to the people that he encounters. It may be that Melvin's dog just died. Or his kid. He may be sick today and trying to stay positive. It may be that his whole life is hell and he's trying to stay positive. It may be that his whole life is hell and he has the charity not to infect others with the negativity that he is probably entitled to. He may simply be being kind.
He might be the most honorable person you've ever met.
There are people who are born with the physiological wiring to be irritated by people. Their internal hardware is constructed to experience pain and despair from every input, and they recognize that pain and despair are not good qualities, and so they consciously and constantly fight their natural forces.
This is hard work, and it is unrelenting. If people with this handicap had to "act natural", they'd just pick up a shotgun and start shooting people. These are the people who exist propped up by the "positive thinking" that untroubled people think is so silly. Without it, naturally irritable people would be looking for a carbon monoxide inhaler or a bridge from which to jump, every day.
People like Melvin are to be admired.
They fight, every day, to be kind to the people who they deal with.
And by the way, as people in general get dumber, generation by generation, more and more of them interpret any cheerful personalities as evil, whether insincere or perfectly natural. I think that Sarah Palin and the hysterical hatred that people have for her fall into this basket. I don't think that Ms Palin is a rat with a smile, but some uncharitable people do, and react with shocking viciousness.