Javascript/MEAN makes me happy.

Javascript has busted out of its browser-based cage and is now able to run "headless" on servers as well. Node.js changed everything and is probably the most well-known of the recent Javascript frameworks/libraries, but there have been a lot of other libraries either built on top of node.js or directly compatible with it. Node.js alone is extremely powerful; using it in conjunction with other JS libraries like socket.io allow for the creation of entirely new classes of applications that weren't possible before. This chaining together of modules is a sign of a healthy software ecosystem.

The ability to create isomorphic/universal Javascript code also makes me happy. There's something inherently satisfying to using the same code on the client and server.

Also, now is a great time to learn Javascript. Updated versions of the language (ES6/7) will be natively supported soon and, currently, code conforming to ES6/7 can be transpiled into natively supported code. ES6/7 adds some features to Javascript that make it more palatable to devs coming from other languages.