Exactly whose feelings does てしまう express?

Question

When Vてしまう is used to express that an action is regrettable or undesirable, does it express the opinion of the speaker or the listener? Put another way, when one says Vてしまう, are they expressing that they feel that V undesirable, or that they are under the impression that their listener will find V undesirable?

For example, would it be appropriate for a man to say to his wife (who he knows does not care for her weight at all),「そんなに食べたら、太っちゃうよ！」if he thinks weight is unsightly? He can't be looking out for his wife's feelings, because he knows that she can't care less about getting fat.

On the other hand, would it be appropriate for a man who is wholly accepting to say 「そんなに食べたら、太っちゃうよ！」 to a wife who has once stated a negative opinion regarding gaining weight? In that case, he wouldn't be saying it out of selfish regard, but for hers.

Answer 1

I'm posting this as an answer only because it might be an edge case in "answer vs comment." (I'm prone to think it's not an answer, because the answer isn't largely about Japanese but it's potentially debatable and downvotable as an explanation if I'm wrong).

The しまう construction expresses the speaker's judgment with respect to the outcome of the verb before しまう. Whether those are "feelings" seems debatable to me and whether that judgment is shared by the listeners would seem to depend wholly on the speech utterance.

I argue this based on the following truism:

Things people say reflect things the people who say them think.

Apart from coerced speech, some really strong empathy, and communicating with children who need all references rearranged to them, what you say is always you.

So then it's pretty clear it is the judgment of the speaker.

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The listener's judgments, however, are incidental to the use of ~しまう because

1. the speaker does not necessarily know these appraisals when they use the construction.
2. the speaker can use it in contexts that expect agreement, are indifferent to agreement, or even that expect the listener to take it as insulting.