Here is my inexpert opinion.

A Shaman is a term to help identify someone by the kind of life they live.  Just like an artist or an outdoor guide, its not so much a profession as it is a predisposition to a way of life.   I don't think one actively goes out to become a shaman.  I feel like its probably one of those things where you live your life a certain way, and one day people start calling you a shaman.  Maybe no one ever gives it a formal title, including yourself, but you just are one by the fact that there is no other good term to describe the way that you live.

People can claim they are shaman, and maybe some of them are, but the act of proclaiming oneself a shaman doesn't mean much.  If anything it probably detracts from itself. 

I believe the universe decides who the shaman will be.  I don't think anyone has to consciously decide to be one, they must just live their life in line with what they know and believe.