I was elected in 2011, the first election using our STV system that we had on a large site. I'm now employed by Stack Exchange, something I'll come back to at the end of my answer.
If you look at my profile, you'll see that I joined Stack Overflow very late in 2008. I'd been reading the site since it opened to the public, just watching - because Jeff was introducing heaps of concepts that, at the time, I though were quite radical if all mixed together. Anyone can basically edit anything? That's not going to go well, I said, as I stuffed my face full of those puffed cheesy things and pulled up a chair. By December, I saw it actually working, and I decided to try answering a few of the softer questions to test the waters. There were harder questions surrounding stuff I knew that I could have answered, but they already had great answers.1
Stack Overflow did a great job of funneling people into doing more things. You either became (sometimes oddly) obsessed with the extrinsic motivation that the site offered, or you became really curious as to how everything ticked. In order to figure out how everything ticked, you also had to become oddly obsessed with the extrinsic motivations the site offered in order to unlock the stuff so you could figure it out. It was brilliant, and it gave me someone intelligent to argue with for free (Jeff) on this planet called 'Uservoice' - I was hooked.
New things quite often don't become old things - that's just the way that selection works when you're competing for people's attention and admiration, especially when they're programmers. What? We're closing questions now? Ah well, it was nice while it lasted - but this time I was eating home-made pork rinds. That was just my first week after signing up. The site obviously continued to exist.
We then proceeded to be awesome. (fast-forward a bit)
I can remember where I was when the Challenger exploded, where I was when the OJ verdict was read, and where I was when I hit 10k rep on SO. I was in IRC, talking to this guy, who also had a penchant for writing lengthy answers and had been neck-and-neck with me on the rep scale.
I promise, I'm not digressing.
Sometimes, really smart people weigh a bunch of arbitrary factors, take a hard look and decide that while they spend their paycheck hours taking care of business, the business of building something that helps everyone else do the same is probably the most convenient way to make things just a little better than they might have been for someone else. They start looking at this amazing .. mousetrap .. and want to start filling the cracks they see.
Everyone has a slightly different motivation. Some want to work on quality, some want to make sure new users have a better experience. Some have been irked continuously for more than a few years about how certain things work and just want a stronger voice to demand fixing. Every single one of us decided that there was something else we should be working on apart from answering questions, and every single one of our communities agreed, once they realized that we wouldn't make a point of bossing people around unless there was a very good reason to do so.
When this happens, you start caring about things more than you did previously. You care about every single post people type on Meta because that's what you've decided to work on. You start feeling a little hurt when people vent their frustrations about an action you took on Reddit. You decide that your best contribution to the site is building something better for people like you.
That's now my full-time job, I went so far as to realize that I'm much better at being a programmer than I am at programming, after 20 years of doing it.
You can substitute 'programmer' with cook, mechanic, linguist, parent, gamer, netizen, chip hacker, poker player - at some point you need an outlet to give differently than you have to that which you're really passionate about. That's why folks become moderators, or come to work here, or flip us the bird and start something aiming to fix everything we're not doing .. right.
It's easy (well, sometimes) to be you. Working on enabling people to be better than you? Yeah, that's a mod. When you get to that point, you begin caring more about getting things out of the way of awesomeness, because awesome really does need to flourish and you've got ideas as to how it can do that - all you need is a chance.
1Many will say that the first year or so was full of nothing but Microsoft stack stuff, that wasn't the case. It was hard, but not exactly rare to find unanswered questions on C / Linux (kernel) which is what I was eating and breathing at the time.