For me, it was PhotoPrism. I used to be an idiot, and used Google Photos as my gallery. I knew that it was terrible for privacy but was too lazy to do anything about it. When Google limited storage for free accounts, I started looking for alternatives. Tried out a lot of stuff, but ended up settling on PhotoPrism.
It does most things that I need, except for multiple user support (it's there in the sponsored version now). It made me learn a bit about Docker. Eventually, I learned how to access it from outside of my home network over Cloudflare tunnel. I'm happy that I can send pics/albums to folks without sharing it to any third party. It's as easy as sending a link.
Now I have around a dozen containers on a local mini pc, and a couple on a VPS. I still route most things through Cloudflare tunnels (lower latency), only the high bandwidth stuff like Jellyfin are routed through a wireguard tunnel through the VPS.
Anyway, how did you get into selfhosting? (The question is mostly meant for non-professionals. But if you're a professional with something interesting to share, you're welcome as well.)