The speaker argues you should actively question beliefs you picked up at different ages instead of keeping them by default. If you don't consciously choose what you believe now, you're letting other people decide for you, which shrinks what you think is possible.
A lot of people haven't chosen or come back in adulthood to reevaluate based on where they are in their current career about what it is they believe. Do you still believe what you believed when you were 8 years old? Do you still believe what you were told by your first partner or your managing physician when you were 22 years old? If you're not consciously choosing what you want in life and what you want to believe, then you've left it to everybody else. You've put that responsibility on them to fill your ideas with what's possible for you and what results are available to you.
