Everybody has a unique genetic makeup (unless they have an identical twin). Different genes lead to different innate physical and mental qualities. At the same time, we all have different personal experiences that affect our development both positively and negatively. The importance of these two factors in determining our potential is debated hotly under the term "nature vs. nurture" and there is a huge amount of research available if you google that term.
It's an easy debate when it comes to elite NFL athletes, however, because if you want to be one of the best RBs in the world you need to have both. There are plenty of hard workers who will never have the genetic gifts necessary to compete at the highest level, and there are plenty of people who inherited elite athleticism but do not have the work ethic, mental discipline, or environment to succeed as a professional athlete.
It's an easy debate when it comes to elite NFL athletes, however, because if you want to be one of the best RBs in the world you need to have both. There are plenty of hard workers who will never have the genetic gifts necessary to compete at the highest level, and there are plenty of people who inherited elite athleticism but do not have the work ethic, mental discipline, or environment to succeed as a professional athlete.