I don't really know if I can say that I have an origin story, since I'm not really in the game (Damn you, Will. (My friend who 'plays' Tau (not really, we never play at all)))
But, if I did, I would have to say that it was when I saw a box with this guy on. The dude was a total badass, and he had a sword, and a gun, and his dudes behind him had completely badass guns that were so coooolll! And, oh GOD, Aliens! Lots of aliens, red ones, running up to the dudes, and they were gonna eat them, it was so awesome, I had to have it, mom can I ple-- WTF, fifty dollars?!
Three or four years later (at least), I got the Battle for Macragge, still with no real idea what the game was, or what I was doing, but at fifteen, I was using this awesome thing know as the internet. Couple hundred dollars later, and I'm still not playing. At all. Damn. I really hate it when this sort of thing happens.