If you're highly motivated and not an Air or Law contract, 9 times out of 10 you want to be an Infantry Officer. What separates and determines, from this large pool of guys who want to be Infantry Officers, who actually gets the opportunity to go to IOC? In other words, what thins out the pack or decides who actually will get to live out there goal of being an Infantry Officer, and who will be left with an MOS secondary to what they wanted?
Most often, it's the TBS student's performance in the field, love for the field, toughness (mental/physical), leadership, and above all else their ability to put other's needs above themselves.