Doesn't it seem like a lot of people who initially show some passive interest in math suddenly become really into math? (I see this a lot more with math than most other subjects - where people who have passive interest in something often lose such interest)
I'm noticing the same thing myself. I'm starting to intensively study math, and I'm noticing that it's pulling my interests deeper in - because there is just so much and it's so consistent (and since I realize that understanding math isn't just about having intelligence). This comes even though I'm determined not to PhD in math (foreign language requirements and the arrogance that mathematicians generally have towards non-mathematical subjects).
Unlike random activities, which have more shallow ones.
This seems more common with theoretical math than competition math though (people who do competition math often seem more into the social aspects of math, which makes them more externally motivated and in tune with the arbitrary accomplishments of others).