We have an impression that a story 'should' go a certain way, towards darkness, while joy and satisfaction give off a feeling of eerieness and illegitimacy.

我们有这样一种印象，一个故事“应该”朝着某种方向发展，也就是黑暗的方向，而快乐和满足却会给人一种怪异和非法的感觉。