Simple: He admires two of the best to ever play.
Following a blistering performance that resulted in an eight-shot victory at the Hyundai Tournament of Champions last week, Spieth told the Associated Press (via the New York Times) he admires Tiger Woods' and Phil Mickelson's career track records as a way to put his achievements in perspective:
The more you look at Tiger, you look at Phil, you start realizing how far away you are from one of the best players to ever play the game. And if you look at that, it keeps your head small.