"You have nothing at all to apologize for," Severus said, shaking his head. Perhaps she had been tempted to smile at seeing his underpants that day, but he had gotten her killed. That rather trumped her apparent amusement, and then some.
He probably shouldn't have agreed to let her hug him, considering that he gave a slight shiver just from feeling her fingers on his hair. But he also had no wish to hurt her feelings. So, when Lily stepped aside so that he could stand and held out her arms ... well, he got to his feet, regardless of his reservations, and stepped into her arms, wrapping his around her back.
Immediately, his eyes began to sting, and he closed them tightly, inhaling with a shudder. And even this could not be put down to the Dementors -- not even close. It had been well over two decades, after all, and he'd never thought to hug her again. "I've missed you, Lily," he whispered, shaking his head slightly with a sniff.