I guess I just really enjoy working on the frontend, on the part of the application that the end user will directly interact with in the end.
I also like that there's no compiling and stuff, and we have tools like BrowserSync / Webpack hot reloading allowing instant feedback during development. Also, Chrome Developer Tools is an amazing set of tools - just to think of the Timeline view and the Profiler. It's great to have these out of the box.
And last but not least - the community. I think there's great discussion around JavaScript and it's moving at an amazing speed, driven by the community, always building on the patterns / practices of the past and trying to improve.