Hmmn..
For me they're not random at all.
Meaning, obsessed with guitar + mixing music, this developed slowly over a few months intense listening, then became a lifelong focus.
Obsession with sex, seemed to come out of nowhere, but really is because a partner I had was extremely sexual, and after a month with her it kinda became my focus.
I feel like I have a choice involved, meaning if I don't obsess with something it will become a person, or a problem, but I have the choice to throw it into music..
Another good example is maths, I didn't focus at all during school, over the past few weeks knowing I'm going back to study I've began thinking about maths every single day.
I think some obsessions can spark suddenly, but they also vanish just as suddenly.
Those that blossom over time can last a lifetime or months/years.
_________________
All hail the new flesh, cause it suits me fine!