I haven't read most of the other replies but I'll give thesea good go with my limited knowledge
1. An old tradition stemming from Religious belief/Plague fears as many have said before. Over the years it simply became a custom and is seen as the polite thing to do, although is appears to be slowly dying out in many places
2. Same as above. Just with the added context of Western culture often being trained to say thankyou for everything nice anyone does for you. This is in stark contrast to Russia (From my understanding) Where saying thankyou is actually reserved for greater acts and they do not understand why Westeners say it for everything that happens ever.
3.There are many terms that could be used instead like "See you later" "Until next time" or even "L4terz!". I would surmise that goodbye simply became the most popular form and has thus adapted it's original meaning and intent into something less final. Nowadays it seems to be a catchall term for leaving anyones presence for any length of time, however long or small. Also acting as a way of officially 'ending' an interaction, this prevents awkward pauses where people are not sure if they should leave or stay or keep talking...
4. Shaking hands I believe descended from a sign of trust and proof that one was not carrying a weapon. It also incorporated the willingness to have physical contact to show you were not afraid of catching disease from them and many other believes from a very long time ago. going a great many centuries back variations of it can be found like arm clasping, clasping arms and kissing on the cheeks etc. I believe over the centuries it became a formal sign of recognition and so became the only 'polite greeting'. Not doing it was then taken as a sign of direct mistrust or insult.
5. No idea. this one seems localized as in many places it is not rude at all. Also elbows on the table helps with eating certain foods.
6. Another descendant of the age of chivalry where men of higher class an morals took on codes of honor to always aid and 'respect' the 'more fragile' feminine gender. It obviously just descended as the Respectful thing to do, although do it for some girls and they will think it is sweet and lovely, do it for others and you'll have to put up with a 20 minute rant about sexism.
7. Pull it out too far and it will be very funny. As for the serious answer see point 6
8. Again see the thing about handshaking. It was a trust thing, but is also HIGHLY cultural. different levels of eye contact is expected in different cultures. what you then do with your eyes can also be a compliment or insult. In some cultures, constant eye contact is taken to mean a direct challenge to the others authority. In others it is simply polite. In others still constant eye contact is a sign of disrespect and arrogance. This is partly why a lot of cultures hate Americans, many other reasons of course but this is often cited as a factor when talking about American rudeness abroad.
9. This seems to me to be more about people not wanting to actually know what is really happening in your life, otherwise they would feel obliged to actually care and do something about it. People want you to smile so they can be selfish and ignore the reality of your feelings.
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1. People just feel it, I could try and explain this to you but explaining empathy to those who cannot understand it is a truly difficult topic and I believe beyond me when I am this tired. a super basic simplifications is that is is an innate urge to feel emotion for the person and their situation, probably activated by mirror neurones triggered by the knowledge or presumption of their emotional or physical state.
2. Many people are arrogant and take things that should not be taken as so, as insults. Others are very insecure so take constructive feedback/criticism as personal attacks. I know when I am tired I am more easily offended but when I am not tired I am actually very hard to offend. Mainly I think it stems from how people view themselves and their self-image. People innately care about this self-image and how they believe themselves to be, so when something challenges that, they lash out rather than having to rethink that self-image. Finally some people are just super touchy for whatever reason and see the world as out to get them so they will be offended by just about anything- for example they would read this and despite it being a general discussion, they would assume that it was actually secretly directed at them and would then rant about it, ignore these people, they simply are not worth anyones time.
4. Almost everything can affect this, I recently became A LOT more organized than I used to be, but I also know that emotional state, amount of sleep, time of year and levels or sunlight can affect this on me. If you are commenting on a long term general trend in a specific populous.... maybe due to people relying more and more on electronic devices to do their thinking and remembering for them to the point where they train their own brains less and so are actually worse at it?
5. Look up the Darwin awards, this is my fav way of praising stupidity, praise those who removed themselves from the gene-pool.
6. Sadly as I spoke about above. It is actually seem as a direct insult now. Not shaking a hand is seen to be you saying they are not worthy of you touching them or you believe them to be dirty and below you. It's cultural, just tell them you have a very bad and possibly infectious skin condition and they wont want to shake yours.
7. They build their own self worth around others attention and you not giving to them harms that. Or they thought they went out of their way to be polite but see you not responding as throwing that back at them. There can be a number of reasons.