The best explanation of fate is that when you face a situation, the actions you would take based on your past character and logic are what constitute fate. If the actions you should take when something happens deviate significantly from your usual character habits, that is called luck. Actions taken habitually without any external conditions are fate, while actions taken after thinking that go against your character are luck. Therefore, luck is change, and fate is constant. The so-called defiance of fate does not mean defying heaven, but rather going against one's own instincts.