As shown by the responses from the Big Ten Champions, most athletes don't specialize in the first sport they do and the main reason they choose to specialize is because they are good at the sport.  A study by  Katherine H. Rizzone, MD, MPH,  proves this;  "the average age of sport specialization is high school 15.1 years); for a majority (62%).  The study also found that 56% of  athletes choose to specialize in their sport because it was the sport they wanted to play/liked the most "  and 30% because  it was sport that they were "best at".