Meaning of んでしょう in this context?


In the sentence 「なんて優しいんでしょう！」what is the purpose of adding んでしょう？




Before addressing んでしょう, it bears noting that

> なんてX｛の／なの｝でしょう！

is itself a common phrase pattern used to express surprise or admiration of
something, where X can be an i-adjective (e.g. かわいい), a na-adjective (e.g.
きれい), or a noun phrase (e.g. 素敵なお方). Examples of how this might translate to
English are exclamatory phrases like "How exciting!" or "What a beautiful
view!"

Some additional examples:

> なんて素晴らしいのでしょう！  
>  How wonderful!
>
> なんて失礼なのでしょう！  
>  How rude!
>
> なんて気が利く人なのでしょう！  
>  What a thoughtful person!

Given the above, 「なんて優しいんでしょう！」 would translate to "How kind!"

With that established, let's break down 「なんて優しいんでしょう！」, and for convenience of
explanation let's normalize the ん to の:

> なんて優しいのでしょう！

なんて is an adverb that expresses feelings of surprise/shock or admiration. 優しい
is an i-adjective, the extent of which is being admired in this utterance.

Now we come to のでしょう. This is the [declarative/explanatory の] + [でしょう
functioning as an expresser of emotion (since it accompanies なんて, a 感動を表す語;
see the following excerpt from 明鏡国語辞典)]:

> でしょ‐・う  
>  連語〔「だろう」の丁寧語〕  
>  ➎《疑問や感動を表す語を伴って》反語や感動を表す。  
>  「だれが信じてくれる━か」  
>  「何と魅惑的な人━」

Quick English translation of the above:

> でしょ‐・う  
>  collocation (of でしょ (mizenkei of です) + う) [polite form of だろう]  
>  5. (when accompanying language that expresses questioning/doubt or emotion)
> serves as an indicator of a rhetorical question or expresses emotion
>
> 「だれが信じてくれるでしょうか」  
>  "Who would believe me?" / "Who would believe that?"
>
> 「何と魅惑的な人でしょう」  
>  "What an enchanting person!"

So the の emphasizes the declaration, followed by でしょう which serves to augment
the emotion expressed by this exclamatory phrase.

