Hmmm when I started the game I never thought I'd do much more than lone wolf it through everything and eventually do everything. Well about 5 months down the line I entered coil for the first time. The story hooked me. I wanted to see more. But I was also having fun, I was with friends(how that happened I still don't know but I've made quite a bunch of friends by then) and we had fun. I never thought of myself as anything more than a decent player so always felt like I was holding people back. But I always kept trying to improve. And that also became a part of my reason to raid. To challenge myself, to push myself to the best I can be.
To sum it up: I raid for friends, fun, story, challenge.