Practice, but a lot of it comes down to talent. Some have it, some do not.
I am not talented. I can hold my own on the track, but I will forever be a middle pack finisher, and I am happy with that. If you are happy to aspire to be the best, DO IT.
But, if it immensely frustrates you to no end that you cannot be the fastest, rethink your priorities, or risk being a very unhappy, insecure person.