Barristan shouldve begged Dany for forgiveness for not protecting her mother from Aerys. He shouldve apologized for serving Robert after he called Dany's niece and nephew "dragonspawn" and smiled at their corpses. He shouldve told her, blatantly, what Aerys did to Rhaella and to others and asked forgiveness for not upkeeping any of his oaths besides "obey the king."
The audacity to stand beside Daenerys and not even begin to approach these topics. To call himself Dany's protector when he refused to protect her mother. To call himself her advisor when he can't even be honest about who her father was, about the role he played in Aerys' cruelty.