Well, maybe I'm a little late with all of this, but I do invite you to read Jono Bacon's great post "The Ubuntu Ethos", as well as Daniel Holbach's take on the subject: "Why I believe in Ubuntu".
As for my part, I'll have to go with the same answers as them. It's just really stimulating to see all the work that we can put together when working as a community. Sure, there is room for improvement, sometimes things are a little ugly, but overall everybody does a great job. In Ubuntu-QC for example, there is a clear motivation to help out people, and that's nice to see. Just about everywhere else, I see the same drive to make a difference, to help out those who hope for something better (by showing them about our wonderful distro!) and to support those who are just having trouble with one or two little details on their system.
I also feel that drive, and it's great to see that people are recognizing it and are happy to see our interest in their software, in Ubuntu, and in free software in general.
And even more importantly, it's nice to have a nice community you can go have a beer with every once in a while, and talk about those weird computer things that interest us.
As David Thomas so nicely put it in his post on the subject, learning about Ubuntu has been the catalyst to doing more, or at least finding out where to start. I had been wanting to participate more in free software, and as I learned about Ubuntu I also learned how I could make a difference, and found a great opportunity to contribute where it matters, where you've got a friendly atmosphere to work in, and where what you can do to help is clear and goals are attainable.