There are tales of Zuko's power and then there are the present state of things. When Iroh is young, he hears his friends make jokes behind the firelord's back and every night he sees how that old man sits on the throne. His back arched over, a cane in hand, gray hair and a withered face.
Just once he questions those legendary tales but then Zuko looks into his eyes and Iroh is scared and excited all at once.
So much fire.