The earliest obsession I can remember is things that plug into each other in general and sound equipment in particular. That was probably when I was 5. When other kids were drawing tree, houses, and dinosaurs, I was drawing system diagrams.
I got obsessed with Star Wars not long after that and I did a lot of Star Wars roll playing with my friends (yes, I used to have friends). Then George Lucas killed it for me with the prequels - DAMN YOU TO HELL, GEORGE LUCAS!! !!
I had an obsession with pipe organs from about 7th grade to 10th grade when I just lost interest for no particular reason.
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~Michael