GRAMMAR DETAIL

でなくてなんだろう

what could it be if not

Uses a rhetorical question to assert emphatically that a particular judgment or label is unquestionably correct.

Examples

互いを思いやるこの気持ちが、友情でなくてなんだろう。

If this concern we show for each other is not friendship, what is?

人を助ける気持ちが優しさでなくてなんだろう。

If the desire to help others is not kindness, what is?

危険を承知で人を救った行為が勇気でなくてなんだろう。

If saving someone despite knowing the danger is not courage, what is?

Structure: Follow an evaluative noun with でなくてなんだろう to assert it emphatically through a rhetorical question implying that no other label fits.
Basic usage: 人を助ける気持ちが優しさでなくてなんだろう。 emphasizes that the desire to help people is kindness itself.

Affirmation through a negative question
- 友情でなくてなんだろう asserts that it is clearly friendship rather than denying that it is friendship. Distinguish it from a request for someone to supply a different factual answer.
Evaluation compared with でしかない
- 友情でしかない can sound limiting, meaning nothing more than friendship. 友情でなくてなんだろう conveys emphatic conviction that friendship is exactly the right description.