Thinking about a situation when one person wants to apologise to another for not living up to his expectations and letting him down, I suddenly realised I don't know how to express it properly in Japanese. The only thing I came up with was 期待はずれ, which wasn't exactly what I wanted.
In fact, 'to live up to (expectations)', 'to let down' and 'to disappoint' are all active verbs, which describe the actions of the person who is actually at fault. We also say it the same way in Russian. However, what I noticed when searching for the Japanese equivalents was that most part of those words and phrases I found were describing not the guilty person himself but the one he offended. The only word close to what I was searching for was 裏切る, which, beyond the usual 'to betray', can also mean 'to disappoint' or 'to let down'. There was also 艶消しにする, but I doubt it's widely used and it's a bit different from what I needed.
On the other hand, not only 'kitaihazure', but all other possible variants were all words regarding not the apologising person but the other party. Even though 期待はずれ means 'disappointment; let-down' if we look it up in the dictionary, it's essentially a noun obviously derivated from はずれる - to miss (the goal). However, instead of saying 期待をはずれる, they say it as a noun to describe the result of actions, not the action itself.
The other variants I found in weblio: 気落ちさせる, 失望させる and がっかりさせる, are all based on the passive verb させる - 'to make', because 'saseru' is essentially an 'auxiliary verb indicating the causative', like jisho.org says. In other words, all these verbs literally mean 'to make a person discouraged', 'to make a person lose his hopes', 'to make a person feel disappointed, dejected; lose heart; feel emotionally drained; feel let down' (jisho).
I wouldn't be surprised if someone wrote a big dissertation on this topic.
