User blog comment:SayuriDarling/The House of Hades Official Discussion/@comment-7153535-20131011000313/@comment-5050355-20131011014403

It was mainly because Nico was alone in Tartarus so there was nothing to distract him from the true horror of the place. Percy had Annabeth.

It could also be that being part-Underworld Nico could see the essence of things.

Or maybe Gaea just wanted him to go crazy so she could capture him easier.