I've pretty much an identical experience; spent ages looking for open source Go projects to contribute to so I could a) improve my Go chops and b) finally contribute back after all these years. The feeling you get when your pull request is accepted with a big thumbs up is unlike anything I've had working as a developer during the day. I think it because you clearly respect the author of the project your contributing too (otherwise you wouldn't use the library in the first place!), and the acknowledgement that you are a "peer".

Just makes you hungry for more!