> Are you sure they don't?
I've seen conversations with or without me on either side of the spectrum, so it's not a given.
> And how do you know?
By writing neutral, simple, kind e-mails and messages to people.
> You have to be understanding too and know when you are wrong too and when other people are right.
Asking something you don't know to someone you never contacted before cannot contain the points you mentioned. If the conversations are ongoing, then things are different. It's important to be able to accept you're wrong and do it humbly. That's also very correct, but sending an email in the tone of "Hello, I'm wondering about this, can anybody point to me to the right direction, because I've failed to achieve it myself." Has neither right, nor wrong in it. Also, it can't contain anything more than appreciation of others time (and you show it by not spamming the person or the list).
> Their behaviour is a completely different matter, until it is not borderline criminal, it should be considered as an expression of their feelings.
Borderline criminality is not a defined international standard and can‘t be put on a scale homogenized throughout all cultures and conversation styles. It's also important to note that while the sending party is not entitled to replying party's kindness, replying party is not entitled to be tolerated for its tone of choice. This is why we have politeness and kindness as an international standard and aspired underpinning of conversations throughout this planet.
One can show their emotions perfectly well without yelling or being rude, too, if wishes to chose that way.
> My doctor shouting to my face to take the f*king medicine, is probably trying to help me.
Then you should first ask yourself what you have done to your doctor to deserve this reaction, maybe?
> Me doctor telling me kind words, knowing very well my lazy ass, is probably not giving a flying f*ck about me.
No. Some people say strong words kindly. They can only tell "You'll just die if you continue doing this." with a calm tone, and you may lose sleep over it for a month. The choice is, of course, yours.
So, as a result, we come to the conclusion of "If both parties act reasonably, then kindness and politeness is a natural outcome".
Then as a corollary, we can ask that "If everyone behaving reasonably, why at least one of the parties is not polite?"