I remember, about a year ago, when I was looking up random videos on YouTube.
As I browsed around, I developed an interest in those "Windows Error Remixes" and stumbled upon one titled "Crazy Windows Error".
In the description, it said the source music was none other than "Marisa Stole The Precious Thing".
Wondering what the hell that was, I decided to look it up...
...And that was my official introduction into Touhou. This was the first Touhou work of any sort that I remember; strangely enough, it's still actually my favorite "silly" PV.
I started encountering other videos like Alice-Dere, and soon enough I discovered the games. Although I had some experience with other shoot-em-ups before, Touhou was my first danmaku-type game. (In particular, I played Einh?nder and ZANAC, both of which are more about shooting skill and resource management than about the usual bullet hell gameplay.)
I first decided to try Touhou 06. On Hard Difficulty (and I can't remember why). I actually managed to make it to the Stage 3 boss battle before I lost my last life. Within about 5 days I was able to get a 1CC of Normal Difficulty, but too bad I didn't know that the Japanese text was asking me to save a replay and I was just pushing buttons to get back to the main menu.
(Oh well, it was a pretty bad and extremely exhausting run because I was relying on almost completely reactive skills that I developed from FPS games.)
Despite that exhausting and fairly poor performance, Touhou is actually what got me interested in danmaku and bullet hell shooters. For a few months, I stopped playing Touhou and explored other shooters like Seihou and Samidare (where I have a score that I'm very happy with). Then, about 6 months ago, I drifted back into Touhou and started actually getting 1CCs on the other games and Extra Stages. In particular, I had fun with Touhou 11 and its Extra Stage.
But even though I still don't play the games often at all, Touhou has pretty much permanently embedded itself in my daily experiences. Like with Team Fortress 2, which I also developed an interest in around the same time, I now enjoy the countless Touhou references that I started noticing almost everywhere I looked.
An interesting fact, though, is that I came into contact with Touhou long before I even realized it. Four years ago, I used to play rhythm games almost exclusively, especially StepMania.
I had this song called "200 BPM Dump" that was a medley of various songs at 200 BPM. At the very end of the medley was this "catchy" tune that I liked, but I didn't give it much thought at the time. Then, as I was browsing YouTube for Touhou soundtracks, I came across this one theme from Touhou 09 that sounded vaguely familiar even though I have never played that game. It turns out that Komachi's theme was the first Touhou song I ever heard, and I didn't even know it for four years.