Severus had never been particularly good at taking advice from anyone. As a result, he snapped at Harry, Minerva, the house elves and anyone else who tried to offer help or suggestions. He even, eventually, snapped at Poppy over the potions he was supposed to take. He was not allowed to brew them himself and he refused to take potions from St Mungos so Draco had stepped in and was brewing them for him.