I have to agree with Phil. When I first started visiting this site, I mostly read, but did post a few questions here and there. But what I found out is that I am passionate about what I do professionally, and that has driven me to be more active on this site. And I believe that is the same for the other "heavy hitters" on this site.

Passion for thier profession, and a desire to help others to be as passionate for thiers. As you learn more about how to use SQL Server to its full potential, the more engaged you become in both your career (profession) and its community.