I am a happy Linux user and have been for a while now. There are so many reasons for this and I’ll try to put as many as I can in as little text as I can.
I have studied Philosophy in college, so I have a kind of “natural affinity” with GNU/Linux for the things it tries to stand for. This plays a huge part of my satisfaction on investing time into Linux (as a user). It is hard to say a lot more about this subject without mentioning other OSes as we live in a world dominated by patents of, in my humble opinion, things that couldn’t have a patent. If 1 + 1 can not have a patent, then, again, IMHO, neither should software. Also, I believe that, at least, public administration should only use public code. If nobody agrees, that’s fine too. I am no “active activist” but that is important to my “happiness” using Linux.
On the other hand, I am very happy with the recent evolution of Linux. A LOT of things progressed quickly and I think it is picking up steam as a desktop OS. Mainly graphics but also audio, input, fonts and so on. I love to read KDE’s weekly usability and productivity blog posts. It makes me value even more the work people are investing into a free and open platform and it is really nice to see the heights they are reaching! As was mentioned, I don’t think Linux needs any customization for a new user. It is just that customizing it is so nice that it feels like it we are loosing opportunity if we don’t… Also, I’ve converted more than 50% of my family (parents, wife, nephews included) to use Linux. I am the “technical support” anyway and when I stopped using other OSes they lost their “free technical support”… Most people only need a browser and an office suite 
As a developer, the platform has always made sense to me, even before I could understand it fluently. I mean, it is the “tinkerer’s” platform. Learning to use the shell, discussing about file systems, installing the OS with plenty of options many times and understanding the network stack made me a better programmer (I believe).
One thing I will disagree with previous answers is that Linux isn’t simple. I really think it is simple. Sometimes simpler than other OSes (but not always). What I agree, though, is that simple does not necessarily mean “easy”. Think about LISP: it has the simplest syntax any language might dream to have (only parentheses mean anything in LISP), though it is not easy to understand its semantics at first contact (even for seasoned programmers). This feels to me like an OO programmer approaching FP and saying it is “too complex”. It is different for sure, but not necessarily more complex. I think Linux is discussed a lot and has more “principles” and “standards” than other OSes and that is why it is simpler to understand. Currently, other OSes feel alien to me and I can’t be productive on them anymore. I guess this is natural once you don’t have to switch OSes on a daily basis (it’s been a decade since I last used another OS on my machines).
People should not trust my opinions though! I am an Emacs user too, so… I like to invest my time into tools that are open (to say the least). Like Erlang and Elixir
