It might have been hormones. But Kristoff probably knew better. Doom would have snapped that either way.
"I don't intend to let people know I've been affected. Let them think what they always think I'm doing." Which was that he was lurking in his castle, scheming nefarious, vile means to destroy their meagerly protected countries. He folded his arms. Honestly, if he had wanted to destroy the world, it would have been done a long time ago.
But then, if he was affected, who else was? The young tenor was quiet and thoughtful, and indeed scheming nefarious things. "I have a few things to investigate, in the meantime."