Dumbledore did not smile.
The gentle smile remained on his face, but the curve of his mouth was no longer soft. He looked at Slughorn—at those dark gray eyes, no longer holding any evasion or guilt—and suddenly understood something. He could stand here today and use "you taught Tom Riddle" as moral leverage because he possessed an informational advantage.
And now, Slughorn stood before him, responding with "why didn't you appoint Snape," because he possessed an informational advantage as well. They were both playing a game with things they knew and the other could not be certain of.
And the outcome of today's game had already been decided when Slughorn spoke that name. He had lost.
"Severus Snape." Dumbledore slowly repeated the name, giving each syllable an even weight. "Ah... he is indeed an exceptionally fine professor, and an exceptionally loyal one. But—" He paused.
"The post of Defense Against the Dark Arts professor has a particular... characteristic. It requires someone with a more public identity, someone students can trust more readily. Professor Snape, at Hogwarts..."
"...Your thoughts are far too complicated." Slughorn finished for him. His tone was so steady that it was as though he had already been given the answer.
"I know... I know. As the head of Slytherin House, you are more concerned that Snape might secretly teach Dark Magic, aren't you? A fine concern, but it still does not explain..." He raised his chin slightly. "...why you are unwilling to let someone capable enough, loyal enough, and already employed at that castle for more than a decade take over this position. Unless..." He drew out that final word a little longer than usual, like a needle slowly being pulled from cloth,
"...what you need is not the most suitable teacher, but the most suitable... pawn."