Well ... I first got into contact with OpenSource due to Gratis: OpenOffice, Firefox etc.
Combining my knowledge of OpenSource with my tendency to break stuff (Reinstalling Boston for the nth time) led me to Linux which I first tinkered with and soon fully adapted.
I had a short hopping phase where I went from Ubuntu (my starter) via Debian (accidentally tried stable) to Arch.
Stuck with arch on my personal machines now run Ubuntu for my work machine and Debian for Servers.
My favourite distro is the right tool for the job (see above) but I'm pretty happy with Arch