这要从fail和disappoint两个词的意思上看。
先说fail，<牛津高阶英语词典>上是这么解释的：
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).
牛津双解词典上的例句：Words fail me, ie I cannot find words (to describe my feelings, etc). 我无法用言语来表达（自己的感情等）.
可以看出，fail是to be unable to help when needed,用在这里是没问题的。意思是说，他的好脾气总是对他有帮助的（当然了也可以翻译成：从来不让他失望）。
再看disapoint：<牛津高阶英语词典>上是这么解释的：
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就是说，正到用的时候脾气不好了，用不上了，使他失望。这与前面的内容显然是矛盾的。
你认为回答的怎么样呢？ 仅仅系统给的20分，我一般不会这样回答的。呵呵，你会给我追加分吗？