When I feel that a character got the ability primarily because the story-writer decided to write about that character, I get the "chosen one" feeling.
These are some factors that would make me feel that way:
a) The ability is unrelated to the inclination, the problems, or wishes the character has before getting the special ability (When this happens, the main character seems like a simulation dummy to entertain a 'what-if' question.)
b) The ability is something that would save another world that the character knew nothing or wouldn't have cared otherwise. (When this happens, it seems like an excuse to include the character in that other world.)
c) The ability is related to a mission that the character would not have cared otherwise. (When this happens, similar to b, it seems like a contrived way to include the character in the mission.)
One formula commonly used to let an outsider be a rising superstar is that the outsider had always been practicing the "foundation skills" of the super ability without realizing it himself. Then, either someone spots his talent, or that he discovers his talent and enters the "competition".
Examples:
1) A chef at a martial arts school who was not allowed to learn how to fight, but started unconsciously using the moves in his cooking and had a good foundation. (When an opportunity comes, allowing him to develop his foundations, he becomes super powerful.)
2) A pizza delivery boy who is always in a rush, always almost get run over by cars and end up just a bit late, without realizing that he has extraordinary reaction time, because he is just trying to be on time for once. (In a different context, holding different tools instead of the handle bars of his delivery bicycle, he becomes a superhero.)
3) A person with a super-computer caliber mind who tries to hide it because whenever people are scared away whenever she solves problems with it. (When she becomes anonymous and realize that he can have a double-life, she start using her ability and become a superheroine.)