Why can some people run quickly?

In my experience I think it was/is because of the following:

-Being a left hander for writing (study the sprinters and find out how many are left-handed)
-Having iron tablets for anaemia when I was 12 (must have done something)
-Generally being scared of the opposition so running like ****

I was generally only quick over about 30 - 70m. The more athletic types can sustain it over longer distances.