Because the power went out.
I was the LJ expert in my group of friends -- I always knew what was going on, why LJ was down, and all that stuff. I kept track of the important communities to track, and I linked people there, and I was on LJ enough that I just knew how to do things. So people would come to me with questions.
I'd seen a few lines somewhere about a volunteer IRC channel. So when the power went out at the data center and LJ went down and stayed down, I got a few frantic IMs from friends. So I found that IRC channel and hopped in.
And I stayed.
I keep volunteering for LJ because I spend a rather lot of my free time here. It'll never replace hanging out with close friends in person, but it augments the good times very well indeed. I've always been the one in the corner with a notebook; I've always shared long and rambling tales about my day at school with Mama. Four pages of paper fit in a standard-sized envelope; after four pages, the rate goes up. I learned that in high school. I had all my friends and we wrote back and forth. It would have been easier with LJ. I still have some of those letters somewhere.
I bought a permanent account when they were on sale. It's been over four years now; it was a good bargain. Not volunteering at this point would frankly feel like freeloading. I want to give something back to the community, and the best way I can do that is by giving LJ my time and attention.
I want LJ to be a good place with spiffy features and a well-informed, caring userbase. I like debating out technical features that have social implications. I like playing Devil's Advocate from time to time. I really treasure watching the moment when someone says "Oh, wow, I get it!" I absolutely love taking an idea that I know a pure programmer would look at and say "No fucking way am I coding that piece of shit!" and hammering on it until that same programmer would be drooling and thinking up ways to make it as elegant and streamlined as possible.
I got started in analysis of the social implications of programming in LiveJournal by summing up current events and reactions in news for some friends. It turned out that I have a bit of an eye for social trends in LJ, a decent way of summing things up, and a very good pen for satire. What I thought was going to be a one-off entry to explain current events wound up getting a lot of comments and translated into Russian. That still amazes me.
I'm fairly sure that it was this work that wound up with me being recommended for the position of junior suggestions maintainer. And recommended I was, and poked and prodded. I'm not sure how many people applied for the position, but I wound up getting it. That's what I do now. I really love getting into the suggestions that probably don't have a chance, and exploring the possibilities. Sometimes this means explaining why they don't have a chance (either technically or socially). Sometimes it means looking around for another way to accomplish the same thing, or diving into the issue at the root of the suggestion and coming out with something that solves the root issue, even though it may be miles away from what the suggestion actually was.
I volunteer for LJ because I'm able to volunteer by doing something I love. It makes LJ a better place, and that makes me happy.