As MindoverMaster said, there are literally thousands of reasons.
I use Windows, Mac, and Linux here and there, but my sweet spot is Linux. I prefer it for reasons that pertain to me and my uses, however I'll share them here. Of course, some of these reasons may be achievable on other platforms, such as Windows or Mac, but believe me when I say, been there, done that, and there's a reason as to why I use what I use.
It's a free operating system.
It's community help is far better than anywhere else I've seen.
It's relatively easy to use (do not confuse familiarity with ease of use).
It's easy to set up a file/web/print server in no time.
It's extremely modular and easy to tinker with the "guts" if need be.
Lots of applications are available cross platform, thereby negating a lot of reasons to use other proprietary platforms.
It's a very secure platform.
I find it requires extremely little maintenance once you have it running the way you need.
I love learning new things. Being on 1 platform alone is boring. Being able to check out Windows, Mac, and countless Linux distributions keeps me busy and allows me to see what other alternatives are out there.