でなくてなんだろう
Meanings
1
if not ... then what is it?; it must be ...; it is definitely ...
usually written using kana alone
A rhetorical expression following a noun or adjective. It strongly asserts that something is exactly that thing, often with a nuance of 'what else could it be?' or 'it's nothing other than ...'. The tone can be emphatic or explanatory.
If this isn't love, what is it? / This is definitely love.
His actions were nothing other than courage.