the interpretation of SHOUTING, is subjective its only shouting if you already know this convention,
Quite true, but the convention predates the invention of the PC. It was developed by the early users of email and of what is now called the internet, back when the content was printed on paper rather than pictured on a screen. These were the same folks who invented emoticons (smileys), which were "shorthand" conventions of juxtaposed characters to simulate smiles, frowns, etc. (If you click on a smiley to put it into your text, you'll see that for most of them the "raw" text consists of just a couple of characters which sort of look like the selected facial expression, e.g., ":" and "D" side by side for the toothy grin.)
why is it more rude to use capital letters than to underline something, or to use italics,....
that has become the accepted convention. And because it was felt that a convention was needed for the purpose. Driving on the right-hand side of the road is no more or less "logical" than driviing on the left-hand side, but it is very important that on a given road everyone follow the same
convention, in order to avoid head-on collisions. So conventions -- laws -- are established. Even though in the US they're opposite to what they are in Britain, they work, because the drivers in each country follow the local law.
[ I] havent learnt to find the underlining button ... perhaps you would be good enough to explain where the thing is,
The "underline" button is the one labelled with a "U" in the row of buttons at the top of the window where you type in your text. This Topic
has some discussion of how to use the buttons. Most work the same way as the "quote" button, though links and images are different. Note that the details of how it works seem to be different on Macs than on PC's. You can read the discussion through to get the details.
Chris Timson tends to use underlining for emphasis. I tend to use italics and sometimes boldface, but I usually avoid underlining, because it is also used to indicate clickable links, and I want to avoid the ambiguity.