Story time, I suppose.
Of course there were the songs. Night of Nights, Bad Apple, and UN Owen Was Her marked my first exposure.
Around my sophomore year of high school (2010) my friend showed EoSD to me. I decided there was too much going on onscreen and decided against getting into it. Not about that.
According to the dates on some pictures I have on my hard drive, this is also when I started downloading Touhou fan art because of whatever reasons 14-year-old me decided.
--Changed schools
In my junior year an artist I liked on DeviantArt did a couple of console personifications. Xbox with green eyes and red hair looked really cool to me, so I tried to find more. I started lurking on the Booru's, and with those two tags learned of Kazami Yuuka. Slightly more interest to find out why she was called the "Dreadnaught-class Sadist". Looked on the wiki and saw the number of characters (around 90 at the time if I remember correctly). Forget that, I'll just look at the art.
Senior year I started playing shmups -- Ether Vapor was one I really got interested in. I was playing it in class on day and another friend (at a different school) showed me... EoSD.
Again, not about it. "Sorry, that looks cool, but I'm not interested"
--Changed cities, schools
Freshman year of college and my roommate mentions something about the music. I blow it off.
Group of friends decide to play Killing Floor and mess around in the workshop, finding the Touhou character mods.
Walk in on my roommate playing Killing Floor with friends with Touhou mods.
Some while later I buy the game and install the mods. I realize we know nothing about these characters, and the only one I knew up to that point (Yuuka) was not in the mod.
"You know what, I'm going to research these characters a bit. Find out more about them. If we're playing them we at least owe them that much" and decide to "sacrifice myself" for the greater good, and read the wiki.
Not about that, too long. What else can I do? Play the games I suppose?
Get the games, play a couple up to stage 2 and just kind of suck at them. I keep getting distracted trying to look at the backgrounds, enemies appearing on screen, music, and quickly die. I drop it again.
Second semester, cleaning my hard drive I decide to play a couple of rounds before I give these files the boot. I manage (with continues) to get to stage 5 in Ten Desires.
I start randomly playing them, all of the windows games, and really getting into it. I don't know when it happened, but I looked up the mangas (official and otherwise), animated micro-series, music, and start to really get in deep.
Too deep, I sometimes think.
(This could technically be the end of the story, since this is really where things took off. The rest is really just me recalling my increasing engagement with it all)
It's the end of spring semester and I'm browsing the wiki "ZUN coming to AWA in September" in the header. Isn't that the guy? Huh. How about that? and mention it to my roommate, who proceeds to loose it. I wasn't sure why.
Over the summer, continuous, gradual exposure. Slightly more into it with each passing month. Then at some point it hits me.
ZUN is coming to the States, wherein I proceed to loose my mind at work.
Fall 2013, I talk with a couple of friends and convince them to go to AWA, one so he can go to the 4chan panel, the other because he just likes cosplaying.
I'm slowly loosing my shit as we drive up there. I'm hyped and don't know why, they're cracking jokes at me, I don't care. These emotions are unusual to me, as I'd never been interested in anything as much as I was now.
Panels, etc.
It's the final day, and we stay late to watch the closing ceremonies. On our way out, I stop to talk to this guy about his "Not-So-Mini Mini-Hakkero" as he called it (it was the size of a basketball, lit up, and apparently the bulb in it got hot enough to light things on fire while not melting the wires/igniting the wooden case. Turns out he was PR for A-Kon in Texas, and wants me and my friends to talk to the manager, who was also there.
Friend taps me on the shoulder "Hey, is that Zun?"
"What makes you say that?"
"The hat, I think"
Look through the window into the parking lot. Shit lost.
PR-guy starts loosing it, too.
Friends convince me to get a picture, and I'm still loosing it. I don't deserve this, I'm not even that into Touhou!
It was at that point I decided I was required to know Touhou. All of it.
Now, I'm going into a local gaming competition in the spring, representing my gaming group in Subterranean Animism, have thought about pitching for a Touhou fan panel at a local con, and frequently get made fun of by other members of the group for being so into Touhou. But like my two other con-goers keep saying:
"Yeah, but you met Zun"
Tl;dr Technically back in my first few years of high school, but I -really- got into it my second semester of college and it grew from there.