Not necessarily the true passion for the particular subject, as much as being highly motivated to pass it. I know some folks who were absolutely average IQ and not truly passionate about anything Uni related (except the idea of finishing it), but simply had firm self-discipline and work-habits instilled into them by parents, as well as this crazy level of competitiveness and ambition that they have to be the best in everything... and frankly they were killing it in all subjects.