先說fail，<牛津高階英語詞典>上是這麽解釋的：
fail---[VN] to DISAPPOINT sb; to be unable to help when needed: When he lost his job, he felt he had failed his family. ◆ She tried to be brave, but her courage failed her. ◆ (figurative) Words fail me (= I cannot express how I feel).
牛津雙解詞典上的例句：Words fail me, ie I cannot find words (to describe my feelings, etc). 我無法用言語來表達（自己的感情等）.
可以看出，fail是to be unable to help when needed,用在這裏是沒問題的。意思是說，他的好脾氣總是對他有幫助的（當然了也可以翻譯成：從來不讓他失望）。
再看disapoint：<牛津高階英語詞典>上是這麽解釋的：
to make sb feel sad because sth that they hope for or expect to happen does not happen or is not as good as they hoped: [VN] Her decision to cancel the concert is bound to disappoint her fans. ◆ I hate to disappoint you, but I'm just not interested. ◆ The movie had disappointed her (= it wasn't as good as she had expected).
可以看出：disapoint是指not as good as one had expected，如果用了disapoint就是說，正到用的時候脾氣不好了，用不上了，使他失望。這與前面的內容顯然是矛盾的。
妳認為回答的怎麽樣呢？ 僅僅系統給的20分，我壹般不會這樣回答的。呵呵，妳會給我追加分嗎？