Yeah, because it's all in the head so it can change very easily compare to the real physical world. But if a personality can be easily changed then it's not max out. Maybe the character can tolerate the ideal that contradict to their ideal, but only to a certain extend. For example: a selfish person can care for other too, but they must always place themselves above all. They can't be selfish and selfless at the same time. If they decides to sacrifice for other, that means they have changed. The character has changed from warrior class to mage class.
How to properly keeping opinion/ideal? Understand other view points, but still keeping your ideal? Wow, this may lead to other study like philosophy and psychology.