I just started my OSS journey but here are some top reasons for me personally:
It's fun! I mean, working on projects in private is fun too, but publicly having that code available (especially for someone who works from home) makes me feel more involved with people.
Job opportunities. I just started and haven't received any job opportunities but I think I'm raising my chances! (Especially because I have no job, haha)
Learning from the community. This is probably my favorite aspect of OSS. I don't have to do things alone. I can seek help and guidance from other people. (To help this even more, I decided to build OSS Buddy, which will be released soon.)
Giving back to the community. I don't have the finances to donate to open source but I do have time. I feel like I'm not just a "consumer" of open source but rather a consumer and provider!
Other small things, like getting quick fixes in for something I rely on in one of my projects or helping to add new features that I would like to see shipped!
To not have to rely on only myself. This means more bug fixes and feature requests shipping!