Comment A matter of perspective (Score 2) 57
In my case it was because I'm a lazy bastard. I needed e bug tracking module (exception details are turned into a unified format to report to a bug tracking server) and I came across one that was already 95% of what I wanted, so I simply contributed enhancements until the final 5% were covered.
The same thing got a former colleague of mine involved in the Firebug project until he became a regular contributor.