That also annoys me, although I must admit I might be guilty of something similar myself. Just the other day my girlfriend called me 'that sort of yeller.' Mind you, luckily she wasn't talking about yelling in ordinary conversation but yelling as a means to communicate quickly throughout the flat. Evidently she would prefer it if I walked into the room she's in to communicate. I think that's just too damn inconvenient. Maybe I'll buy one of those baby-alarms or something.
Anyway in normal conversation I think the tendency to yell usually manifests itself in insecure people. It usually stems from an insecurity and fear of not being heard or ignored. Ironically too, because it often ensures people ignoring you. If you really want to make people listen to you, you should either present a good argument as the Panda suggests or even whisper. It's a weird thing but people usually listen to whispers.
Of course the internet equivalent of yelling is to write needlessly long posts.
