I guess I first stumbled upon Marisa Stole the Precious Thing (in another video, not the actual flash) and McRoll (ugh it needs to die) a few years back, but it didn't really catch my interest. Some time later, I was looking up Ragnarok vids and I saw a sprite edit dance gimmick and I realized, "Hey, it's that song!", so I looked up the real flash for MSTPT. I got hooked.
Then just this summer (Eastern summer, not Western summer), on a whim, I decided to look up Touhou. I found it on Wikipedia, YT and Touhou Wiki. Since the latest entry then was UFO, I downloaded that and... my head asplode.
Since UFO at the time was too hard for me, I got SWR (not knowing UNL was already released). It was nice, but got boring after a while, so I looked again for more of the shmups and apparently, UFO was "the hardest" in the opinion of many (well, SA is harder IMO, but whatever), and IN was the easiest, so I got that aaaaand yeah now I have every game except for StB.