Nobody has a choice about math in K-12 school, though there are kids who will choose the more advanced classes on their own. For instance my daughter proactively convinced her high school to let her skip a grade in math.
In college, you can choose to major in math, or in a math intensive subject like CS or physics. Within those disciplines, you can choose the more mathematical specialties. In the work world, you can volunteer for assignments that involve math, or get a reputation for being willing to solve hard math problems. That's me.
I don't see it as a "switch" because my interest in math was evident (so I'm told) before I could even talk.
Speaking personally, a lot of people would probably say that I'm "good at math" in the sense that I got through a fairly rigorous engineering program and that I've never had issues with the quantitative side of business or other such pursuits. On the other hand, I've never personally considered myself a "mathematician" in the sense that anything approaching upper-level university math was something that came remotely naturally.