As long as it doesn't affect your life in an adverse way, an "obsession" is harmless, and is not pathological.
It's "harmful" if, say, an obsession with Gregorian Chant makes you forget to go to work today.
If one cannot talk about anything else other than their obsession, and don't listen to other speakers while speaking of their obsession, this is often harmful, too.
I remember, as about an eight-year-old, that all I wanted to talk about was my "special interest" at the time. I did not bother to listen to what others were saying. Very quickly, I was ostracized and became the object of scorn and bullying.