In simple text with no formatting available the only way to draw attention to a particular word or phrase is to do what you did and put it in capitals, thus drawing attention to it. Putting an entire post in capitals is drawing attention to everything you have to say - HEY GUYS, LOOK AT THIS. That is shouting. Also it's harder to read.

Without soundwaves to signify an increase in volume we must rely on sight, bigger letters seem to beg more attention.

Well, I can tell you that I was on Usenet since the early 80's and it was always considered shouting there. I would suspect that this is not an Internet or computer created idea, but actually originated from books. By the early 20th century, at least, all caps have been used in novels to convey shouting. Comic books are another example. Though they are usually all caps anyway, larger and bolder print is used for shouting, yelling, screaming, etc. Back in the early days of computers before ASCII was developed and fully supported, the only choice was all caps. After ASCII, caps provided a new way to show emphasis, something we'd never been able to do before. Also, this was well before rich text format had been developed, so no italics, underscores, etc. Also, all caps is also just plain hard to read, so I really think that computer users pretty much started using lower case as soon as it was developed, since it's much easier on the eyes, and fell back on using all caps only for emphasis. The term ITSELF was probably coined and surely gained its widespread use on the Net and it has become one of the basic rules of Netiquette. I really believe, though, that the concept predates the web by a good long while.

Hi Scooter-libee: I don't know the origin but the various sites I've visited, it was always pointed out on them as shouting when someone posted in caps.