Or they might be 'rebelling' against their parents, especially if the parents were strict. And some people were never taught to be considerate and polite.
A lot of people are in physical pain and it sometimes makes them very 'intolerant', short-tempered, etc.
And even more people are suffering from 'emotional' pain;
- their conscience is bothering them about something they've done and they feel bad,
- they don't feel good about themselves (usually as a result of being abused as a child), etc.
I'll often read a 'romance' novel where the 'hero' doesn't say please or thank you when he speaks to the heroine. It sounds like he's telling her what to do rather than asking her. And the strongest example of this (I've observed) is in a lot of bdsm novels where the guy is rude and disrespectful to the woman. The best 'master'/someone in a position of power is someone who earns the respect of those who serve him. One way to do this is to speak to others with politeness, the way we would like to be spoken to.
When people are rude to us, it's very tempting to respond in the same way but it often escalates the problem: then they feel justified in responding in a rude manner to us. But if we respond by being firm but polite, if often helps to diffuse the situation. At the very least, we get to demonstrate politeness.
When I read Lauriesa's post, I didn't know Lauriesa was talking about a member of PTS.