New guy here. Hoping I can still post in this topic.
For me, there are a few reasons why I've become obsessed with Touhou:
1. The characters.
No form of media has provided characters I'm so interested in. The characters are surprisingly open to interpretation, which has its good and bad points. Such are the reasons why I've become so entranced with Flandre, Alice, and Koishi, for instance.
2. The music.
I got into Touhou by listening to arranges from Crow's Claw. After a while, I decided to try out EoSD, and...yeah. I've never seen a series of games with such glorious music.
3. The fanbase.
All the different fangames, stories, and music offer so much that the games leave open. Excluding certain character interpretations that I could easily do without (Psychotic serial killer RANDOM Flandre, for instance), the fans have provided a veritable goldmine of opportunity.