One analysis is if we take someone who is used to delving into their beliefs and ideas, second guessing, testing them, changing them or fleshing them out as needed. They may even entertain ideas they don't actually hold or some they are vehemently against, just for the sake of mental exercise.

Such a person might be less likely to feel confident in the ideas they hold because they know how much potential for growth and change there is in that realm. By extension they would be less likely to be impressed with themselves.

Basically, humility?