...when you spend four years of your life developing a Touhou Fanfic idea.
That, and when you make it a habit to play Touhou tracks on the piano in the college lounge between breaks despite not being not that good with the piano, just so that you can snare any passing Touhou fan to your presence.
Edit:
Also, when you start thinking of cars as danmaku when you cross the street. There's this nagging voice in my head that keeps on telling me to 'graze for points' every time I cross the street.