He hears familiar footsteps behind him. He sniffs and wipes his eyes briskly with his sleeve.
"I thought that I might find you here, Severus."
He does not bother turning around.
"And why did you think to find me at all, Headmaster?" He can already guess the answer.
"Hagrid told me that you seemed troubled. I was concerned."
He stares into Lily's eyes and sees his reflection. He looks different in the green light, a bit more stalwart, a little less pathetic.
Slowly, he steps back from the mirror, until the only reflection that could be seen was himself, in the dim moonlight. Snape watches Dumbledore slowly come up behind him. The older man raises a hand and motions to touch Snape's shoulder - but he thinks better of it. The sharp blue eyes look somewhat wistful as they gaze into the mirror.
"You must pardon me," Dumbledore says, "but I must ask, what is it that you see in Erised?"
When Saul of Tarsus rode towards Damascus with torture and pursuit in his mind, the heavens opened and a light blinded him. He was brought to his knees and he became a new man.
How could he explain this to Dumbledore? Cold and calculating Dumbledore, who asked "And what will you give me in return?", would he understand Damascus?
When he turns and faces the old man, his eyes are clear.
"Transfiguration," he says.
Dumbledore holds his gaze and this time Snape does not look away.
"Minerva always said you had an aptitude for it."