Iroh has never sought anyones aproval. Though his Father has always smiled down on him.
Thus, it saddens him to see Ozai so determind to win it.
Their Father isn't a very good parent though, and while he favors Iroh, he barely acknowledges Ozai's existence.
Iroh knows good and well that its a sore spot for the boy.
Sore spots are dangerous, he knows, if they are constanly poke at, they will become bruises, then cuts, and slowly will fester 'till nothing is left but poison.
He fears what his brother will be as an adult.