I've been asked this question many times but most often I conclude by saying that it has something to do with Free Software Foundation and Open source movement.
The easiest way I've been able to answer this is:
It's free and open source - logical, simple software management. once you've used Linux you realize what a bloated, disjointed mess Windows is by comparison.
High flexibility. If you want to build a GUI environment from the ground up, you can go Archlinux. If you're looking more for something that covers your needs out of the box, there are plenty of options like Mint and Ubuntu for that.
Using Linux for a few months ( @ initial stage) probably taught me more about how computers work than years of using Windows. Windows seems to try and protect users from themselves a lot of times, which I'm sure is a good idea for the majority of people but sucks for people who want more control over their OS.
"If something is not working on Windows, it's many times not your fault. If something is not working on Linux, it's probably your fault."