The Last Gasp, by Telanu
"You cannot hide, Severus," Dumbledore says; "I can see it in your eyes. More than your eyes. You are very ill."
Snape's face is more sallow than usual and the circles under his eyes are deep and he hasn't been eating much. "Don't be ridiculous," he sneers. "What do you think -- " a snort -- "that I'm dying of love for the boy?"
Dumbledore regards him compassionately. "Of course," he says. "That's what people who love him do, after all."
ETA: After posting, I realized this was no doubt inspired by Luthien's fantastic (and very sad) fic The Rain Keeps Falling, and the discussions she and I had afterwards. Go, go, read!