We simply don't live long enough to know everything. So, we have a bunch of beliefs. Things based on what people who we trust have told us, and rough guesses and theories based on the way we see the world around us working.
But what happens when you suddenly find out that, perhaps for a very large chunk of your life, you were wrong. Undeniably wrong. Perhaps you just choose to carry on, and try to find another reason to support what you now know is wrong - perhaps even subconsciously (confabulation). This may especially be the case if it is really hard to change your mind. Maybe you have build a career around that false belief. Maybe your whole social network still holds the belief, and to reject it is to cast yourself into the wilderness.
Or, you do what Stephen Hawking did and you change your mind when facts present themselves differently. Admittedly, being a scientist - there is no harm in admitting you were wrong. You are not cast out. There is no excommunication for changing your mind.
'On the mountains of truth you can never climb in vain: either you will reach a point higher up today, or you will be training your powers so that you will be able to climb higher tomorrow.'- Nietzche
I guess it boils down to who you surround yourself with. It is tempting to say that if they are worth their salt - people should celebrate self-criticism and taking another step up the mountain. It is hard though. We invest a lot of emotional energy in our beliefs. Perhaps we only have capacity/courage to change our minds so often.