I think at the end of the day I want to be shipping things to people, and frontend is where you feel that the most. It's where a lot of hard work (from the backend, etc) can be brought together into a real product, and that's exciting. Having knowledge of all the parts of the system helps me build better UIs and understand the tradeoffs (optimistic updating, how to pass data back to the server, etc)
But it's an apt question because I do have a feeling that I might change my focus at some point. I am interesting in machine learning and graphics programming, so we'll see if I can make that into an actual job.
There's also the fact that it's just so practical to work on frontends. There is so much work and good jobs out there. I used to do a lot of graphics development but I wasn't interested at all in getting into the games industry. So part of it is practicality.